pack-bitmap.c: continue looping when first MIDX bitmap is found

In "open_midx_bitmap()", we do a loop with the MIDX(es) in repo, when
the first one has been found, then will break out by a "return"
directly.

But actually, it's better to continue the loop until we have visited
both the MIDX in our repository, as well as any alternates (along with
_their_ alternates, recursively).

The reason for this is, there may exist more than one MIDX file in
a repo. The "multi_pack_index" struct is actually designed as a singly
linked list, and if a MIDX file has been already opened successfully,
then the other MIDX files will be skipped and left with a warning
"ignoring extra bitmap file." to the output.

The discussion link of community:

  https://public-inbox.org/git/YjzCTLLDCby+kJrZ@nand.local/

Helped-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Teng Long <dyroneteng@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack-bitmap.c

@@ -506,15 +506,16 @@ static int open_pack_bitmap(struct repository *r,
 static int open_midx_bitmap(struct repository *r,
 			    struct bitmap_index *bitmap_git)
 {
+	int ret = -1;
 	struct multi_pack_index *midx;
 
 	assert(!bitmap_git->map);
 
 	for (midx = get_multi_pack_index(r); midx; midx = midx->next) {
 		if (!open_midx_bitmap_1(bitmap_git, midx))
-			return 0;
+			ret = 0;
 	}
-	return -1;
+	return ret;
 }
 
 static int open_bitmap(struct repository *r,