git-commit: initialize TMP_INDEX just to be sure.

We rely on TMP_INDEX variable to decide if we are doing a partial commit,
as it is only set in the partial commit codepath.  But the variable is
never initialized.  A stray environment variable from outside could
ruin the day.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-commit.sh

@@ -25,6 +25,7 @@ refuse_partial () {
 	exit 1
 }
 
+TMP_INDEX=
 THIS_INDEX="$GIT_DIR/index"
 NEXT_INDEX="$GIT_DIR/next-index$$"
 rm -f "$NEXT_INDEX"