Introduce the config variable pack.packSizeLimit

Introduce the config variable pack.packSizeLimit

"git pack-objects" has the option --max-pack-size to limit the file
size of the packs to a certain amount of bytes.  On platforms where
the pack file size is limited by filesystem constraints, it is easy
to forget this option, and this option does not exist for "git gc"
to begin with.

So introduce a config variable to set the default maximum, but make
this overrideable by the command line.

Suggested by Tor Arvid Lund.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>