repack: silence warnings when auto-enabled bitmaps cannot be built

repack: silence warnings when auto-enabled bitmaps cannot be built

Depending on various config options, a full repack may not be able to
build a reachability bitmap index (e.g., if pack.packSizeLimit forces us
to write multiple packs). In these cases pack-objects may write a
warning to stderr.

Since 36eba0323d (repack: enable bitmaps by default on bare repos,
2019-03-14), we may generate these warnings even when the user did not
explicitly ask for bitmaps. This has two downsides:

  - it can be confusing, if they don't know what bitmaps are

  - a daemonized auto-gc will write this to its log file, and the
    presence of the warning may suppress further auto-gc (until
    gc.logExpiry has elapsed)

Let's have repack communicate to pack-objects that the choice to turn on
bitmaps was not made explicitly by the user, which in turn allows
pack-objects to suppress these warnings.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>