GC should not pack objects only referenced by ORIG_HEAD,...

There are references which are returned by
RefDatabase.getAdditionalRefs() which are allowed to point to
non-existing objects. These refs should not save objects from being
garbage collected. Examples for these references are ORIG_HEAD,
MERGE_HEAD, FETCH_HEAD and CHERRY_PICK_HEAD. Native git will not take
these references into account when doing a gc and therefore these
references may point to non-existing objects after a gc. Teach JGit's
GC to behave the same: ignore additional refs if they don't start with
"refs/". Examples for refs returned by getAdditionalRefs() which do
start with "refs/" are the bootstrap refs when using reftree's (see
commit 115f1ad3974d1162b33d1c8eff466019d1f249f3). See also
http://article.gmane.org/gmane.comp.version-control.git/294126.

Bug: 479697
Change-Id: I10e40589f13e72aacdd9f86f3b44696fd1cd068a
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsGarbageCollector.java

@@ -241,7 +241,12 @@ public class DfsGarbageCollector {
 		if (!addl.isEmpty()) {
 			List<Ref> all = new ArrayList<>(refs.size() + addl.size());
 			all.addAll(refs);
-			all.addAll(addl);
+			// add additional refs which start with refs/
+			for (Ref r : addl) {
+				if (r.getName().startsWith(Constants.R_REFS)) {
+					all.add(r);
+				}
+			}
 			return all;
 		}
 		return refs;

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/GC.java

@@ -655,8 +655,12 @@ public class GC {
 	}
 
 	/**
-	 * Returns a collection of all refs and additional refs (e.g. FETCH_HEAD,
-	 * MERGE_HEAD, ...)
+	 * Returns a collection of all refs and additional refs.
+	 *
+	 * Additional refs which don't start with "refs/" are not returned because
+	 * they should not save objects from being garbage collected. Examples for
+	 * such references are ORIG_HEAD, MERGE_HEAD, FETCH_HEAD and
+	 * CHERRY_PICK_HEAD.
 	 *
 	 * @return a collection of refs pointing to live objects.
 	 * @throws IOException
@@ -668,7 +672,12 @@ public class GC {
 		if (!addl.isEmpty()) {
 			List<Ref> all = new ArrayList<>(refs.size() + addl.size());
 			all.addAll(refs);
-			all.addAll(addl);
+			// add additional refs which start with refs/
+			for (Ref r : addl) {
+				if (r.getName().startsWith(Constants.R_REFS)) {
+					all.add(r);
+				}
+			}
 			return all;
 		}
 		return refs;