git-commit: make sure we protect against races.
git-commit: make sure we protect against races.

An earlier commit 8098a178b26dc7a158d129a092a5b78da6d12b72
accidentally lost race protection from git-commit command.
This commit reinstates it.  When something else updates HEAD
pointer while you were editing your commit message, the command
would notice and abort the commit.

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