Fix git-runstatus for repositories containing a file named HEAD

Fix git-runstatus for repositories containing a file named HEAD

The wt_status_print_updated() and wt_status_print_untracked() routines
call setup_revisions() with 'HEAD' being the reference to the tip of the
current branch. However, setup_revisions() gets confused if the branch
also contains a file named 'HEAD' resulting in a fatal error.

Instead, don't pass an argv to setup_revisions() at all; simply give it no
arguments, and make 'HEAD' the default revision.

Bug noticed by Rocco Rutte <pdmef@gmx.net>.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>