ObjectDirectory: fix closing of obsolete packs

This resolves a regression introduced in fef78212.

Change-Id: Ibb4521635a87012520566efc70870c59d11be874
Signed-off-by: Marc Strapetz <marc.strapetz@syntevo.com>

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

@@ -911,9 +911,10 @@ public class ObjectDirectory extends FileObjectDatabase {
 
 			final String packName = base + PACK.getExtension();
 			final File packFile = new File(packDirectory, packName);
-			final PackFile oldPack = forReuse.remove(packName);
+			final PackFile oldPack = forReuse.get(packName);
 			if (oldPack != null
 					&& !oldPack.getFileSnapshot().isModified(packFile)) {
+				forReuse.remove(packName);
 				list.add(oldPack);
 				continue;
 			}