gitweb: Don't use Content-Encoding: header in git_snapshot

gitweb: Don't use Content-Encoding: header in git_snapshot

Do not use Content-Encoding: HTTP header in git_snapshot, using
instead type according to the snapshot type (compression type).
Some of web browser take Content-Encoding: to be _transparent_
also for downloading, and store decompressed file (with incorrect
compression suffix) on download.

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