Scan loose ref before packed in case gc about to remove the loose

Before this change, jgit used to read packed-refs before scanning
loose refs. That was not a problem if gc didn't run concurrently. When
gc did run concurrently with such refs reading, that order sometimes
broke the latter. This lead to reading an older version of a ref's
tip, which meant "losing" the real tip or commit. The specific
read-Vs-gc concurrency scenario which broke reading that way follows:

1. let ref R be in packed-refs and R' be in loose
2. jgit starts reading packed-refs
3. gc also starts its business around that very time
4. jgit still has the time to read R from packed-refs
5. as gc is not done yet updating packed-refs with R'
6. jgit then starts scanning loose refs (or is about to)
7. gc quickly ends up being done moving loose R' to packed-refs
8. so gc (quickly) removes loose refs
9. -while jgit is scanning loose refs, now gone
10. so jgit assumes no loose to consider => packed-refs winning
11. so jgit wrongfully returns R (from 4.) as the tip, instead of R'.

This fix switches the order so loose refs are scanned (secured) before
taking the time to read packed-refs. This way, knowledge of the
likelier tip is guaranteed for ref reading to return the true tip
- despite concurrent gc. If there is no loose ref to scan, jgit reads
packed-refs and lands on R' (or S), which it then returns, as
expected. The gerrit issue [1] should be solved by this fix.

[1] https://code.google.com/p/gerrit/issues/detail?id=2302

Change-Id: Ibd120120a361a3a6ed565f3836afc1db706fbcdd
Signed-off-by: Marco Miller <marco.miller@ericsson.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

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

@@ -312,11 +312,10 @@ public class RefDirectory extends RefDatabase {
 
 	@Override
 	public Map<String, Ref> getRefs(String prefix) throws IOException {
-		final RefList<Ref> packed = getPackedRefs();
 		final RefList<LooseRef> oldLoose = looseRefs.get();
-
 		LooseScanner scan = new LooseScanner(oldLoose);
 		scan.scan(prefix);
+		final RefList<Ref> packed = getPackedRefs();
 
 		RefList<LooseRef> loose;
 		if (scan.newLoose != null) {