scripts: do not get confused with HEAD in work tree

scripts: do not get confused with HEAD in work tree

When you have a file called HEAD in your work tree, many commands that
our scripts feed "HEAD" to would complain about the rev vs path
ambiguity.  A solution is to form command line more carefully by
appending -- to them, which makes it clear that we mean HEAD rev not
HEAD file.

This patch would apply to maint.

Signed-off-by: Junio C Hamano <gitster@pobox.com>