git-clean fails on files beginning with a dash

Reproducible with:

$ git init-db
$ echo "some text" >-file
$ git clean
Removing -file
rm: invalid option -- l
Try `rm --help' for more information.

Signed-off-by: Dennis Stosberg <dennis@stosberg.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-clean.sh

@@ -19,8 +19,8 @@ ignored=
 ignoredonly=
 cleandir=
 quiet=
-rmf="rm -f"
+rmf="rm -f --"
-rmrf="rm -rf"
+rmrf="rm -rf --"
 rm_refuse="echo Not removing"
 echo1="echo"