PackExt: Add a reverse index extension.

There is no reverse index PackExt because the reverse index is not currently
written to a file. This prevents fine-grained performance reporting for reverse
indexes, which will be useful when introducing a reverse index file and
observing performance changes.

Add a reverse index extension that matches the one in cgit
(9bf691b78c/Documentation/gitformat-pack.txt (L302))
in preparation for adding a reverse index file while observing
performance before and after.

Change-Id: Iee53f1e01cf645a3c468892fcf97c8444f9a784a
Signed-off-by: Anna Papitto <annapapitto@google.com>

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/pack/PackExt.java

@@ -27,7 +27,10 @@ public enum PackExt {
 	BITMAP_INDEX("bitmap"), //$NON-NLS-1$
 
 	/** A reftable file. */
-	REFTABLE("ref"); //$NON-NLS-1$
+	REFTABLE("ref"), //$NON-NLS-1$
+
+	/** A pack reverse index file extension. */
+	REVERSE_INDEX("rev"); //$NON-NLS-1$
 
 	private final String ext;