pack-objects: warn on split packs disabling bitmaps

pack-objects: warn on split packs disabling bitmaps

It can be tempting for a server admin to want a stable set of
long-lived packs for dumb clients; but also want to enable bitmaps
to serve smart clients more quickly.

Unfortunately, such a configuration is impossible; so at least warn
users of this incompatibility since commit 21134714 (pack-objects:
turn off bitmaps when we split packs, 2014-10-16).

Tested the warning by inspecting the output of:

	make -C t t5310-pack-bitmaps.sh GIT_TEST_OPTS=-v

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>