git-p4: explicitly specify that HEAD is a revision

'git p4 rebase' fails with the following message if there is a file
named HEAD in the current directory:

	fatal: ambiguous argument 'HEAD': both revision and filename
	Use '--' to separate paths from revisions, like this:
	'git <command> [<revision>...] -- [<file>...]'

Take the suggestion above and explicitly state that HEAD should be
treated as a revision.
Signed-off-by: NVlad Dogaru <vdogaru@ixiacom.com>
Acked-by: NPete Wyckoff <pw@padd.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-p4.py

@@ -3086,7 +3086,7 @@ def rebase(self):
         print "Rebasing the current branch onto %s" % upstream
         oldHead = read_pipe("git rev-parse HEAD").strip()
         system("git rebase %s" % upstream)
-        system("git diff-tree --stat --summary -M %s HEAD" % oldHead)
+        system("git diff-tree --stat --summary -M %s HEAD --" % oldHead)
         return True
 
 class P4Clone(P4Sync):