gitweb: supply '-n' to gzip for identical output

For projects that do not release official archives, gitweb's snapshot
feature would be an excellent alternative, and but without the '-n'
('--no-name') argument, gzip includes a timestamp in output which results
in different files.  Because some systems hash/checksum downloaded files
to ensure integrity of the tarball (e.g FreeBSD), it is desirable to
produce tarballs in a reproducible way for that purpose.

Whilst '--no-name' is more descriptive, the long version of the flag is
not supported on all systems.  In particular, OpenBSD does not appear to
support it.

Supply '-n' to gzip to exclude timestamp from output and produce idential
output every time.

Signed-off-by: Fraser Tweedale <redacted>
Acked-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>