git-log: make sure we have some commit to start from.

When no usable head/tag is specified, git log barfed with
underlying error message from rev-list, which was not helpful.

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

git-log-script

@@ -1,3 +1,5 @@
 #!/bin/sh
 . git-sh-setup-script || die "Not a git archive"
+revs=$(git-rev-parse --revs-only --default HEAD "$@")
+[ "$revs" ] || die "No HEAD ref"
 git-rev-list --pretty $(git-rev-parse --default HEAD "$@") | LESS=-S ${PAGER:-less}