git-commit: make sure we protect against races.

An earlier commit 8098a178b2
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 <junkio@cox.net>

git-commit.sh

@@ -595,12 +595,14 @@ then
 		PARENTS=$(git-cat-file commit HEAD |
 			sed -n -e '/^$/q' -e 's/^parent /-p /p')
 	fi
+	current=$(git-rev-parse --verify HEAD)
 else
 	if [ -z "$(git-ls-files)" ]; then
 		echo >&2 Nothing to commit
 		exit 1
 	fi
 	PARENTS=""
+	current=
 fi
 
 {