RevertCommand: use only first line in revert commit message

C git uses only the first line of the title paragraph of the reverted
commit to build the title of the revert commit. Align the JGit behavior
with that.

Since git 2.43.0, a revert of a revert uses a title "Reapply "xxx""
instead of "Revert "Revert "xxx""".[1] This is _not_ implemented in
this change.

[1] https://github.com/git/git/commit/883cb1b8f86d

Bug: jgit-117
Change-Id: I030092c6b9447bb738e6d761af5ce50df58cc6d3

4 files changed, 45 insertions, 9 deletions

diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/api/RevertCommandTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/api/RevertCommandTest.java
index 4ebe994ef7..afd6708d21 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/api/RevertCommandTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/api/RevertCommandTest.java
@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2011, Robin Rosenberg and others
+ * Copyright (C) 2011, 2024 Robin Rosenberg and others
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Distribution License v. 1.0 which is available at
@@ -59,7 +59,9 @@ public class RevertCommandTest extends RepositoryTestCase {
 			writeTrashFile("a",
 					"first line\nsecond line\nthird line\nfourth line\n");
 			git.add().addFilepattern("a").call();
-			RevCommit fixingA = git.commit().setMessage("fixed a").call();
+			// Commit message with a non-empty second line on purpose
+			RevCommit fixingA = git.commit().setMessage("fixed a\nsecond line")
+					.call();
 
 			writeTrashFile("b", "first line\n");
 			git.add().addFilepattern("b").call();
@@ -78,7 +80,8 @@ public class RevertCommandTest extends RepositoryTestCase {
 					+ "This reverts commit " + fixingA.getId().getName() + ".\n";
 			assertEquals(expectedMessage, revertCommit.getFullMessage());
 			assertEquals("fixed b", history.next().getFullMessage());
-			assertEquals("fixed a", history.next().getFullMessage());
+			assertEquals("fixed a\nsecond line",
+					history.next().getFullMessage());
 			assertEquals("enlarged a", history.next().getFullMessage());
 			assertEquals("create b", history.next().getFullMessage());
 			assertEquals("create a", history.next().getFullMessage());
diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/revwalk/RevCommitParseTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/revwalk/RevCommitParseTest.java
index 6872289a8b..2955516af0 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/revwalk/RevCommitParseTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/revwalk/RevCommitParseTest.java
@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2008-2009, Google Inc. and others
+ * Copyright (C) 2008, 2024 Google Inc. and others
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Distribution License v. 1.0 which is available at
@@ -408,6 +408,7 @@ public class RevCommitParseTest extends RepositoryTestCase {
 		final RevCommit c = create(msg);
 		assertEquals(msg, c.getFullMessage());
 		assertEquals(msg, c.getShortMessage());
+		assertEquals(msg, c.getFirstMessageLine());
 	}
 
 	@Test
@@ -415,6 +416,7 @@ public class RevCommitParseTest extends RepositoryTestCase {
 		final RevCommit c = create("\n");
 		assertEquals("\n", c.getFullMessage());
 		assertEquals("", c.getShortMessage());
+		assertEquals("", c.getFirstMessageLine());
 	}
 
 	@Test
@@ -423,6 +425,7 @@ public class RevCommitParseTest extends RepositoryTestCase {
 		final RevCommit c = create(shortMsg);
 		assertEquals(shortMsg, c.getFullMessage());
 		assertEquals(shortMsg, c.getShortMessage());
+		assertEquals(shortMsg, c.getFirstMessageLine());
 	}
 
 	@Test
@@ -432,6 +435,7 @@ public class RevCommitParseTest extends RepositoryTestCase {
 		final RevCommit c = create(fullMsg);
 		assertEquals(fullMsg, c.getFullMessage());
 		assertEquals(shortMsg, c.getShortMessage());
+		assertEquals(shortMsg, c.getFirstMessageLine());
 	}
 
 	@Test
@@ -441,6 +445,7 @@ public class RevCommitParseTest extends RepositoryTestCase {
 		final RevCommit c = create(fullMsg);
 		assertEquals(fullMsg, c.getFullMessage());
 		assertEquals(shortMsg, c.getShortMessage());
+		assertEquals("This is a", c.getFirstMessageLine());
 	}
 
 	@Test
@@ -450,6 +455,7 @@ public class RevCommitParseTest extends RepositoryTestCase {
 		final RevCommit c = create(fullMsg);
 		assertEquals(fullMsg, c.getFullMessage());
 		assertEquals(shortMsg, c.getShortMessage());
+		assertEquals("This is a", c.getFirstMessageLine());
 	}
 
 	@Test
@@ -461,6 +467,7 @@ public class RevCommitParseTest extends RepositoryTestCase {
 		final RevCommit c = create(fullMsg);
 		assertEquals(fullMsg, c.getFullMessage());
 		assertEquals(shortMsg, c.getShortMessage());
+		assertEquals(shortMsg, c.getFirstMessageLine());
 	}
 
 	@Test
@@ -480,6 +487,7 @@ public class RevCommitParseTest extends RepositoryTestCase {
 		assertEquals(author, p.getAuthorIdent());
 		assertEquals(committer, p.getCommitterIdent());
 		assertEquals("Test commit", p.getShortMessage());
+		assertEquals("Test commit", p.getFirstMessageLine());
 		assertEquals(src.getMessage(), p.getFullMessage());
 	}
 
@@ -494,6 +502,7 @@ public class RevCommitParseTest extends RepositoryTestCase {
 		final RevCommit c = create(fullMsg);
 		assertEquals(fullMsg, c.getFullMessage());
 		assertEquals(shortMsg, c.getShortMessage());
+		assertEquals("This fixes a", c.getFirstMessageLine());
 	}
 
 	private static ObjectId id(String str) {
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/api/RevertCommand.java b/org.eclipse.jgit/src/org/eclipse/jgit/api/RevertCommand.java
index 855c3b1cf3..6643c83662 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/api/RevertCommand.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/api/RevertCommand.java
@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2010, Christian Halstrick <christian.halstrick@sap.com> and others
+ * Copyright (C) 2010, 2024 Christian Halstrick <christian.halstrick@sap.com> and others
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Distribution License v. 1.0 which is available at
@@ -143,8 +143,8 @@ public class RevertCommand extends GitCommand<RevCommit> {
 				merger.setCommitNames(new String[] {
 						"BASE", ourName, revertName }); //$NON-NLS-1$
 
-				String shortMessage = "Revert \"" + srcCommit.getShortMessage() //$NON-NLS-1$
-						+ "\""; //$NON-NLS-1$
+				String shortMessage = "Revert \"" //$NON-NLS-1$
+						+ srcCommit.getFirstMessageLine() + '"';
 				String newMessage = shortMessage + "\n\n" //$NON-NLS-1$
 						+ "This reverts commit " + srcCommit.getId().getName() //$NON-NLS-1$
 						+ ".\n"; //$NON-NLS-1$
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/revwalk/RevCommit.java b/org.eclipse.jgit/src/org/eclipse/jgit/revwalk/RevCommit.java
index 743a8ccce0..55ddebf288 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/revwalk/RevCommit.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/revwalk/RevCommit.java
@@ -1,6 +1,6 @@
 /*
- * Copyright (C) 2008-2009, Google Inc.
- * Copyright (C) 2008, Shawn O. Pearce <spearce@spearce.org> and others
+ * Copyright (C) 2008, 2009 Google Inc.
+ * Copyright (C) 2008, 2024 Shawn O. Pearce <spearce@spearce.org> and others
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Distribution License v. 1.0 which is available at
@@ -524,6 +524,30 @@ public class RevCommit extends RevObject {
 	}
 
 	/**
+	 * Parse the commit message and return its first line, i.e., everything up
+	 * to but not including the first newline, if any.
+	 *
+	 * @return the first line of the decoded commit message as a string; never
+	 *         {@code null}.
+	 * @since 7.2
+	 */
+	public final String getFirstMessageLine() {
+		int msgB = RawParseUtils.commitMessage(buffer, 0);
+		if (msgB < 0) {
+			return ""; //$NON-NLS-1$
+		}
+		int msgE = msgB;
+		byte[] raw = buffer;
+		while (msgE < raw.length && raw[msgE] != '\n') {
+			msgE++;
+		}
+		if (msgE > msgB && msgE > 0 && raw[msgE - 1] == '\r') {
+			msgE--;
+		}
+		return RawParseUtils.decode(guessEncoding(buffer), buffer, msgB, msgE);
+	}
+
+	/**
 	 * Determine the encoding of the commit message buffer.
 	 * <p>
 	 * Locates the "encoding" header (if present) and returns its value. Due to