git-log: make sure we have some commit to start from.
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 <69652caca27c8b940640ad396ab71f93cacec34f@osdl.org>
Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git