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>

git-commit.sh

@@ -26,7 +26,7 @@ refuse_partial () {
 }
 
 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 () {

t/t7500-commit.sh

@@ -93,4 +93,36 @@ test_expect_success 'commit message from file should override template' '
 	commit_msg_is "standard input msg<unknown>"
 '
 
+test_expect_success 'using alternate GIT_INDEX_FILE (1)' '
+
+	cp .git/index saved-index &&
+	(
+		echo some new content >file &&
+	        GIT_INDEX_FILE=.git/another_index &&
+		export GIT_INDEX_FILE &&
+		git add file &&
+		git commit -m "commit using another index" &&
+		git diff-index --exit-code HEAD &&
+		git diff-files --exit-code
+	) &&
+	cmp .git/index saved-index >/dev/null
+
+'
+
+test_expect_success 'using alternate GIT_INDEX_FILE (2)' '
+
+	cp .git/index saved-index &&
+	(
+		rm -f .git/no-such-index &&
+		GIT_INDEX_FILE=.git/no-such-index &&
+		export GIT_INDEX_FILE &&
+		git commit -m "commit using nonexistent index" &&
+		test -z "$(git ls-files)" &&
+		test -z "$(git ls-tree HEAD)"
+
+	) &&
+	cmp .git/index saved-index >/dev/null
+
+'
+
 test_done