archive-tar: use OS_CODE 3 (Unix) for internal gzip

gzip(1) encodes the OS it runs on in the 10th byte of its output. It
uses the following OS_CODE values according to its tailor.h [1]:

        0 - MS-DOS
        3 - UNIX
        5 - Atari ST
        6 - OS/2
       10 - TOPS-20
       11 - Windows NT

The gzip.exe that comes with Git for Windows uses OS_CODE 3 for some
reason, so this value is used on practically all supported platforms
when generating tgz archives using gzip(1).

Zlib uses a bigger set of values according to its zutil.h [2], aligned
with section 4.4.2 of the ZIP specification, APPNOTE.txt [3]:

         0 - MS-DOS
         1 - Amiga
         3 - UNIX
         4 - VM/CMS
         5 - Atari ST
         6 - OS/2
         7 - Macintosh
         8 - Z-System
        10 - Windows NT
        11 - MVS (OS/390 - Z/OS)
        13 - Acorn Risc
        16 - BeOS
        18 - OS/400
        19 - OS X (Darwin)

Thus the internal gzip implementation in archive-tar.c sets different
OS_CODE header values on major platforms Windows and macOS.  Git for
Windows uses its own zlib-based variant since v2.20.1 by default and
thus embeds OS_CODE 10 in tgz archives.

The tar archive for a commit is generated consistently on all systems
(by the same Git version).  The OS_CODE in the gzip header does not
influence extraction.  Avoid leaking OS information and make tgz
archives constistent and reproducable (with the same Git and libz
versions) by using OS_CODE 3 everywhere.

At least on macOS 12.4 this produces the same output as gzip(1) for the
examples I tried:

   # before
   $ git -c tar.tgz.command='git archive gzip' archive --format=tgz v2.36.0 | shasum
   3abbffb40b7c63cf9b7d91afc682f11682f80759  -

   # with this patch
   $ git -c tar.tgz.command='git archive gzip' archive --format=tgz v2.36.0 | shasum
   dc6dc6ba9636d522799085d0d77ab6a110bcc141  -

   $ git archive --format=tar v2.36.0 | gzip -cn | shasum
   dc6dc6ba9636d522799085d0d77ab6a110bcc141  -

[1] https://git.savannah.gnu.org/cgit/gzip.git/tree/tailor.h
[2] https://github.com/madler/zlib/blob/master/zutil.h
[3] https://pkware.cachefly.net/webdocs/casestudies/APPNOTE.TXT

Helped-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

archive-tar.c

@@ -463,6 +463,9 @@ static const char internal_gzip_command[] = "git archive gzip";
 static int write_tar_filter_archive(const struct archiver *ar,
 				    struct archiver_args *args)
 {
+#if ZLIB_VERNUM >= 0x1221
+	struct gz_header_s gzhead = { .os = 3 }; /* Unix, for reproducibility */
+#endif
 	struct strbuf cmd = STRBUF_INIT;
 	struct child_process filter = CHILD_PROCESS_INIT;
 	int r;
@@ -473,6 +476,10 @@ static int write_tar_filter_archive(const struct archiver *ar,
 	if (!strcmp(ar->filter_command, internal_gzip_command)) {
 		write_block = tgz_write_block;
 		git_deflate_init_gzip(&gzstream, args->compression_level);
+#if ZLIB_VERNUM >= 0x1221
+		if (deflateSetHeader(&gzstream.z, &gzhead) != Z_OK)
+			BUG("deflateSetHeader() called too late");
+#endif
 		gzstream.next_out = outbuf;
 		gzstream.avail_out = sizeof(outbuf);