midx: implement `midx_locate_pack()`

The multi-pack index API exposes a `midx_contains_pack()` function that
takes in a string ending in either ".idx" or ".pack" and returns whether
or not the MIDX contains a given pack corresponding to that string.

There is no corresponding function to locate the position of a pack
within the MIDX's pack order (sorted lexically by pack filename).

We could add an optional out parameter to `midx_contains_pack()` that is
filled out with the pack's position when the parameter is non-NULL. To
minimize the amount of fallout from this change, instead introduce a new
function by renaming `midx_contains_pack()` to `midx_locate_pack()`,
adding that output parameter, and then reimplementing
`midx_contains_pack()` in terms of it.

Future patches will make use of this new function.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

midx.c

@@ -428,7 +428,8 @@ static int cmp_idx_or_pack_name(const char *idx_or_pack_name,
 	return strcmp(idx_or_pack_name, idx_name);
 }
 
-int midx_contains_pack(struct multi_pack_index *m, const char *idx_or_pack_name)
+int midx_locate_pack(struct multi_pack_index *m, const char *idx_or_pack_name,
+		     uint32_t *pos)
 {
 	uint32_t first = 0, last = m->num_packs;
 
@@ -439,8 +440,11 @@ int midx_contains_pack(struct multi_pack_index *m, const char *idx_or_pack_name)
 
 		current = m->pack_names[mid];
 		cmp = cmp_idx_or_pack_name(idx_or_pack_name, current);
-		if (!cmp)
+		if (!cmp) {
+			if (pos)
+				*pos = mid;
 			return 1;
+		}
 		if (cmp > 0) {
 			first = mid + 1;
 			continue;
@@ -451,6 +455,11 @@ int midx_contains_pack(struct multi_pack_index *m, const char *idx_or_pack_name)
 	return 0;
 }
 
+int midx_contains_pack(struct multi_pack_index *m, const char *idx_or_pack_name)
+{
+	return midx_locate_pack(m, idx_or_pack_name, NULL);
+}
+
 int prepare_multi_pack_index_one(struct repository *r, const char *object_dir, int local)
 {
 	struct multi_pack_index *m;

midx.h

@@ -70,7 +70,10 @@ struct object_id *nth_midxed_object_oid(struct object_id *oid,
 					struct multi_pack_index *m,
 					uint32_t n);
 int fill_midx_entry(struct repository *r, const struct object_id *oid, struct pack_entry *e, struct multi_pack_index *m);
-int midx_contains_pack(struct multi_pack_index *m, const char *idx_or_pack_name);
+int midx_contains_pack(struct multi_pack_index *m,
+		       const char *idx_or_pack_name);
+int midx_locate_pack(struct multi_pack_index *m, const char *idx_or_pack_name,
+		     uint32_t *pos);
 int prepare_multi_pack_index_one(struct repository *r, const char *object_dir, int local);
 
 /*