repack: disable bitmaps-by-default if .keep files exist

repack: disable bitmaps-by-default if .keep files exist

Bitmaps aren't useful with multiple packs, and users with
.keep files ended up with redundant packs when bitmaps
got enabled by default in bare repos.

So detect when .keep files exist and stop enabling bitmaps
by default in that case.

Wasteful (but otherwise harmless) race conditions with .keep files
documented by Jeff King still apply and there's a chance we'd
still end up with redundant data on the FS:

  https://public-inbox.org/git/20190623224244.GB1100@sigill.intra.peff.net/

v2: avoid subshell in test case, be multi-index aware

Fixes: 36eba0323d3288a8 ("repack: enable bitmaps by default on bare repos")
Signed-off-by: Eric Wong <e@80x24.org>
Helped-by: Jeff King <peff@peff.net>
Reported-by: Janos Farkas <chexum@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>