midx.c: write MIDX filenames to strbuf

To ask for the name of a MIDX and its corresponding .rev file, callers
invoke get_midx_filename() and get_midx_rev_filename(), respectively.
These both invoke xstrfmt(), allocating a chunk of memory which must be
freed later on.

This makes callers in pack-bitmap.c somewhat awkward. Specifically,
midx_bitmap_filename(), which is implemented like:

    return xstrfmt("%s-%s.bitmap",
                   get_midx_filename(midx->object_dir),
                   hash_to_hex(get_midx_checksum(midx)));

this leaks the second argument to xstrfmt(), which itself was allocated
with xstrfmt(). This caller could assign both the result of
get_midx_filename() and the outer xstrfmt() to a temporary variable,
remembering to free() the former before returning. But that involves a
wasteful copy.

Instead, get_midx_filename() and get_midx_rev_filename() take a strbuf
as an output parameter. This way midx_bitmap_filename() can manipulate
and pass around a temporary buffer which it detaches back to its caller.

That allows us to implement the function without copying or open-coding
get_midx_filename() in a way that doesn't leak.

Update the other callers of get_midx_filename() and
get_midx_rev_filename() accordingly.

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

midx.h

@@ -48,8 +48,8 @@ struct multi_pack_index {
 #define MIDX_WRITE_BITMAP_HASH_CACHE (1 << 3)
 
 const unsigned char *get_midx_checksum(struct multi_pack_index *m);
-char *get_midx_filename(const char *object_dir);
-char *get_midx_rev_filename(struct multi_pack_index *m);
+void get_midx_filename(struct strbuf *out, const char *object_dir);
+void get_midx_rev_filename(struct strbuf *out, struct multi_pack_index *m);
 
 struct multi_pack_index *load_multi_pack_index(const char *object_dir, int local);
 int prepare_midx_pack(struct repository *r, struct multi_pack_index *m, uint32_t pack_int_id);