pack-bitmap: avoid use of static bitmap_writer

The pack-bitmap machinery uses a structure called 'bitmap_writer' to
collect the data necessary to write out .bitmap files. Since its
introduction in 7cc8f97108 (pack-objects: implement bitmap writing,
2013-12-21), there has been a single static bitmap_writer structure,
which is responsible for all bitmap writing-related operations.

In practice, this is OK, since we are only ever writing a single .bitmap
file in a single process (e.g., `git multi-pack-index write --bitmap`,
`git pack-objects --write-bitmap-index`, `git repack -b`, etc.).

However, having a single static variable makes issues like data
ownership unclear, when to free variables, what has/hasn't been
initialized unclear.

Refactor this code to be written in terms of a given bitmap_writer
structure instead of relying on a static global.

Note that this exposes the structure definition of the bitmap_writer at
the pack-bitmap.h level. We could work around this by, e.g., forcing
callers to declare their writers as:

    struct bitmap_writer *writer;
    bitmap_writer_init(&bitmap_writer);

and then declaring `bitmap_writer_init()` as taking in a double-pointer
like so:

    void bitmap_writer_init(struct bitmap_writer **writer);

which would avoid us having to expose the definition of the structure
itself. This patch takes a different approach, since future patches
(like for the ongoing pseudo-merge bitmaps work) will want to modify the
innards of this structure (in the previous example, via pseudo-merge.c).

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

builtin/pack-objects.c

@@ -1245,6 +1245,7 @@ static void write_pack_file(void)
 	uint32_t nr_remaining = nr_result;
 	time_t last_mtime = 0;
 	struct object_entry **write_order;
+	struct bitmap_writer bitmap_writer;
 
 	if (progress > pack_to_stdout)
 		progress_state = start_progress(_("Writing objects"), nr_result);
@@ -1339,8 +1340,9 @@ static void write_pack_file(void)
 				    hash_to_hex(hash));
 
 			if (write_bitmap_index) {
-				bitmap_writer_set_checksum(hash);
-				bitmap_writer_build_type_index(
+				bitmap_writer_init(&bitmap_writer);
+				bitmap_writer_set_checksum(&bitmap_writer, hash);
+				bitmap_writer_build_type_index(&bitmap_writer,
 					&to_pack, written_list, nr_written);
 			}
 
@@ -1358,11 +1360,16 @@ static void write_pack_file(void)
 				strbuf_addstr(&tmpname, "bitmap");
 				stop_progress(&progress_state);
 
-				bitmap_writer_show_progress(progress);
-				bitmap_writer_select_commits(indexed_commits, indexed_commits_nr, -1);
-				if (bitmap_writer_build(&to_pack) < 0)
+				bitmap_writer_show_progress(&bitmap_writer,
+							    progress);
+				bitmap_writer_select_commits(&bitmap_writer,
+							     indexed_commits,
+							     indexed_commits_nr,
+							     -1);
+				if (bitmap_writer_build(&bitmap_writer, &to_pack) < 0)
 					die(_("failed to write bitmap index"));
-				bitmap_writer_finish(written_list, nr_written,
+				bitmap_writer_finish(&bitmap_writer,
+						     written_list, nr_written,
 						     tmpname.buf, write_bitmap_options);
 				write_bitmap_index = 0;
 				strbuf_setlen(&tmpname, tmpname_len);