git-p4: explicitly specify that HEAD is a revision

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: Vlad Dogaru <vdogaru@ixiacom.com>
Acked-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>