"git fmt-merge-msg" SIGSEGV

"git fmt-merge-msg" SIGSEGV

Ok, this is a _really_ stupid case, and I don't think it matters, but hey,
we should never SIGSEGV.

Steps to reproduce:

	mkdir duh
	cd duh
	git init-db
	git-fmt-merge-msg < /dev/null

will cause a SIGSEGV in cmd_fmt_merge_msg(), because we're doing a
strncmp() with a NULL current_branch.

And yeah, it's an insane schenario, and no, it doesn't really matter. The
only reason I noticed was that a broken version of my "git pull" into an
empty directory would cause this.

This silly patch just replaces the SIGSEGV with a controlled exit with an
error message.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>