Make GIT_INDEX_FILE apply to git-commit

Currently, when committing, git-commit ignore the value of
GIT_INDEX_FILE, and always use $GIT_DIR/index. This patch
fix it.

Signed-off-by: Rémi Vanicat <vanicat@debian.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/git-commit.sh b/git-commit.sh
index ab43217..fdaa019 100755
--- a/git-commit.sh
+++ b/git-commit.sh
@@ -26,7 +26,7 @@
 }
 
 TMP_INDEX=
-THIS_INDEX="$GIT_DIR/index"
+THIS_INDEX="${GIT_INDEX_FILE:-$GIT_DIR/index}"
 NEXT_INDEX="$GIT_DIR/next-index$$"
 rm -f "$NEXT_INDEX"
 save_index () {