update-server-info: avoid needless overwrites

update-server-info: avoid needless overwrites

Do not change the existing info/refs and objects/info/packs
files if they match the existing content on the filesystem.
This is intended to preserve mtime and make it easier for dumb
HTTP pollers to rely on the If-Modified-Since header.

Combined with stdio and kernel buffering; the kernel should be
able to avoid block layer writes and reduce wear for small files.

As a result, the --force option is no longer needed.  So stop
documenting it, but let it remain for compatibility (and
debugging, if necessary).

v3: perform incremental comparison while generating to avoid
    OOM with giant files.  Remove documentation for --force.

Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>