repository.c: replace hold_locked_index() with repo_hold_locked_index()

hold_locked_index() assumes the index path at $GIT_DIR/index. This is
not good for places that take an arbitrary index_state instead of
the_index, which is basically everywhere except builtin/.

Replace it with repo_hold_locked_index(). hold_locked_index() remains
as a wrapper around repo_hold_locked_index() to reduce changes in builtin/

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/merge.c b/merge.c
index 91008f7..dbbc9d9 100644
--- a/merge.c
+++ b/merge.c
@@ -58,7 +58,7 @@ int checkout_fast_forward(struct repository *r,
 
 	refresh_index(r->index, REFRESH_QUIET, NULL, NULL, NULL);
 
-	if (hold_locked_index(&lock_file, LOCK_REPORT_ON_ERROR) < 0)
+	if (repo_hold_locked_index(r, &lock_file, LOCK_REPORT_ON_ERROR) < 0)
 		return -1;
 
 	memset(&trees, 0, sizeof(trees));