Resolve ~ with no trailing number as the first parent commit

This would previously throw a RevisionSyntaxException

Change-Id: I42b4988c7f6c6454e2ebda13914260e25ac1a889

org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/RepositoryResolveTest.java

@@ -111,6 +111,11 @@ public class RepositoryResolveTest extends SampleDataRepositoryTestCase {
 		assertEquals("1203b03dc816ccbb67773f28b3c19318654b0bc8",db.resolve("49322bb17d3acc9146f98c97d078513228bbf3c0~2").name());
 		assertEquals("bab66b48f836ed950c99134ef666436fb07a09a0",db.resolve("49322bb17d3acc9146f98c97d078513228bbf3c0~3").name());
 		assertEquals("bab66b48f836ed950c99134ef666436fb07a09a0",db.resolve("49322bb17d3acc9146f98c97d078513228bbf3c0~03").name());
+		assertEquals("6e1475206e57110fcef4b92320436c1e9872a322",db.resolve("49322bb17d3acc9146f98c97d078513228bbf3c0~").name());
+		assertEquals("1203b03dc816ccbb67773f28b3c19318654b0bc8",db.resolve("49322bb17d3acc9146f98c97d078513228bbf3c0~~").name());
+		assertEquals("bab66b48f836ed950c99134ef666436fb07a09a0",db.resolve("49322bb17d3acc9146f98c97d078513228bbf3c0~~~").name());
+		assertEquals("1203b03dc816ccbb67773f28b3c19318654b0bc8",db.resolve("49322bb17d3acc9146f98c97d078513228bbf3c0~~1").name());
+		assertEquals("1203b03dc816ccbb67773f28b3c19318654b0bc8",db.resolve("49322bb17d3acc9146f98c97d078513228bbf3c0~~~0").name());
 	}
 
 	@Test

org.eclipse.jgit/src/org/eclipse/jgit/lib/Repository.java

@@ -496,14 +496,17 @@ public abstract class Repository {
 					if (!Character.isDigit(rev[l]))
 						break;
 				}
-				String distnum = new String(rev, i + 1, l - i - 1);
 				int dist;
-				try {
-					dist = Integer.parseInt(distnum);
-				} catch (NumberFormatException e) {
-					throw new RevisionSyntaxException(
-							JGitText.get().invalidAncestryLength, revstr);
-				}
+				if (l - i > 1) {
+					String distnum = new String(rev, i + 1, l - i - 1);
+					try {
+						dist = Integer.parseInt(distnum);
+					} catch (NumberFormatException e) {
+						throw new RevisionSyntaxException(
+								JGitText.get().invalidAncestryLength, revstr);
+					}
+				} else
+					dist = 1;
 				while (dist > 0) {
 					RevCommit commit = (RevCommit) ref;
 					if (commit.getParentCount() == 0) {