Fix ResolveMerger issue with submodule conflict

ResolveMerger throws a MissingObjectException when it encounters
a submodule conflict while merging. The reason is that it treats
the submodule link as a blob and tries to read its contents.

We solve the issue by detecting before content merge whether the
path to be merged is a submodule link, and skip the content
merge if it is.

Bug: 389238
Change-Id: I9a58dfc7716b28a21f5c04cf3a865091ae8dfe7e
Signed-off-by: Tommi Siivola <tommi.siivola@eficode.com>

org.eclipse.jgit/src/org/eclipse/jgit/merge/ResolveMerger.java

@@ -538,6 +538,15 @@ public class ResolveMerger extends ThreeWayMerger {
 			if (isWorktreeDirty(work))
 				return false;
 
+			// Don't attempt to resolve submodule link conflicts
+			if (isGitLink(modeO) || isGitLink(modeT)) {
+				add(tw.getRawPath(), base, DirCacheEntry.STAGE_1, 0, 0);
+				add(tw.getRawPath(), ours, DirCacheEntry.STAGE_2, 0, 0);
+				add(tw.getRawPath(), theirs, DirCacheEntry.STAGE_3, 0, 0);
+				unmergedPaths.add(tw.getPathString());
+				return true;
+			}
+
 			MergeResult<RawText> result = contentMerge(base, ours, theirs);
 			File of = writeMergedFile(result);
 			updateIndex(base, ours, theirs, result, of);
@@ -770,6 +779,10 @@ public class ResolveMerger extends ThreeWayMerger {
 		return mode != 0 && !FileMode.TREE.equals(mode);
 	}
 
+	private static boolean isGitLink(final int mode) {
+		return FileMode.GITLINK.equals(mode);
+	}
+
 	@Override
 	public ObjectId getResultTreeId() {
 		return (resultTree == null) ? null : resultTree.toObjectId();