archive: make zip compression level independent from core git

archive: make zip compression level independent from core git

zlib_compression_level is the compression level used for git's object store.
It's 1 by default, which is the fastest setting.  This variable is also used
as the default compression level for ZIP archives created by git archive.

For archives, however, zlib's own default of 6 is more appropriate, as it's
favouring small size over speed -- archive creation is not that performance
critical most of the time.

This patch makes git archive independent from git's internal compression
level setting.  It affects invocations of git archive without explicitly
specified compression level option, only.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>