Add git config for conflict style merge/diff3

Add a constant in ConfigConstants, and a ConflictStyle enum in
MergeCommand.

Change-Id: Idf8e036b6b6953bec06d6923a39e5ff30c2da562
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>

org.eclipse.jgit/src/org/eclipse/jgit/api/MergeCommand.java

@@ -86,6 +86,20 @@ public class MergeCommand extends GitCommand<MergeResult> {
 
 	private ProgressMonitor monitor = NullProgressMonitor.INSTANCE;
 
+	/**
+	 * Values for the "merge.conflictStyle" git config.
+	 *
+	 * @since 5.12
+	 */
+	public enum ConflictStyle {
+
+		/** "merge" style: only ours/theirs. This is the default. */
+		MERGE,
+
+		/** "diff3" style: ours/base/theirs. */
+		DIFF3
+	}
+
 	/**
 	 * The modes available for fast forward merges corresponding to the
 	 * <code>--ff</code>, <code>--no-ff</code> and <code>--ff-only</code>

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

@@ -397,8 +397,16 @@ public final class ConfigConstants {
 	/** The "ff" key */
 	public static final String CONFIG_KEY_FF = "ff";
 
+	/**
+	 * The "conflictStyle" key.
+	 *
+	 * @since 5.12
+	 */
+	public static final String CONFIG_KEY_CONFLICTSTYLE = "conflictStyle";
+
 	/**
 	 * The "checkstat" key
+	 *
 	 * @since 3.0
 	 */
 	public static final String CONFIG_KEY_CHECKSTAT = "checkstat";