archive: support compression levels beyond 9

Compression programs like zip, gzip, bzip2 and xz allow to adjust the
trade-off between CPU cost and size gain with numerical options from -1
for fast compression and -9 for high compression ratio.  zip also
accepts -0 for storing files verbatim.  git archive directly support
these single-digit compression levels for ZIP output and passes them to
filters like gzip.

Zstandard additionally supports compression level options -10 to -19, or
up to -22 with --ultra.  This *seems* to work with git archive in most
cases, e.g. it will produce an archive with -19 without complaining, but
since it only supports single-digit compression level options this is
the same as -1 -9 and thus -9.

Allow git archive to accept multi-digit compression levels to support
the full range supported by zstd.  Explicitly reject them for the ZIP
format, as otherwise deflateInit2() would just fail with a somewhat
cryptic "stream consistency error".

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

archive.h

@@ -36,6 +36,7 @@ const char *archive_format_from_filename(const char *filename);
 
 #define ARCHIVER_WANT_COMPRESSION_LEVELS 1
 #define ARCHIVER_REMOTE 2
+#define ARCHIVER_HIGH_COMPRESSION_LEVELS 4
 struct archiver {
 	const char *name;
 	int (*write_archive)(const struct archiver *, struct archiver_args *);