IndexDiff: close SubmoduleWalk and use already loaded DirCache

If the SubModuleWalk isn't closed its TreeWalk's ObjectReader won't
be closed. Re-loading the DirCache during an IndexDiff is not only
inefficient but could also give strange results if an external
process had modified the index in the meantime: file diffs would
be based on a "before" state, but submodule diffs on an "after"
state.

Change-Id: Iab948c08ac342138b37263c9028d80b84101f6d6
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>

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

@@ -536,7 +536,8 @@ public class IndexDiff {
 
 		if (ignoreSubmoduleMode != IgnoreSubmoduleMode.ALL) {
 			IgnoreSubmoduleMode localIgnoreSubmoduleMode = ignoreSubmoduleMode;
-			SubmoduleWalk smw = SubmoduleWalk.forIndex(repository);
+			try (SubmoduleWalk smw = new SubmoduleWalk(repository)) {
+				smw.setTree(new DirCacheIterator(dirCache));
 				while (smw.next()) {
 					try {
 						if (localIgnoreSubmoduleMode == null)
@@ -558,12 +559,13 @@ public class IndexDiff {
 								modified.add(subRepoPath);
 								recordFileMode(subRepoPath, FileMode.GITLINK);
 							} else if (ignoreSubmoduleMode != IgnoreSubmoduleMode.DIRTY) {
-							IndexDiff smid = submoduleIndexDiffs.get(smw
+								IndexDiff smid = submoduleIndexDiffs
-									.getPath());
+										.get(smw.getPath());
 								if (smid == null) {
 									smid = new IndexDiff(subRepo,
 											smw.getObjectId(),
-										wTreeIt.getWorkingTreeIterator(subRepo));
+											wTreeIt.getWorkingTreeIterator(
+													subRepo));
 									submoduleIndexDiffs.put(subRepoPath, smid);
 								}
 								if (smid.diff()) {
@@ -577,13 +579,14 @@ public class IndexDiff {
 										continue;
 									}
 									modified.add(subRepoPath);
-								recordFileMode(subRepoPath, FileMode.GITLINK);
+									recordFileMode(subRepoPath,
+											FileMode.GITLINK);
 								}
 							}
 						} else if (missingSubmodules.remove(subRepoPath)) {
-						// If the directory is there and empty but the submodule
+							// If the directory is there and empty but the
-						// repository in .git/modules doesn't exist yet it isn't
+							// submodule repository in .git/modules doesn't
-						// "missing".
+							// exist yet it isn't "missing".
 							File gitDir = new File(
 									new File(repository.getDirectory(),
 											Constants.MODULES),
@@ -598,6 +601,7 @@ public class IndexDiff {
 						}
 					}
 				}
+			}
 
 		}