builtin/pack-objects.c: respect 'pack.preferBitmapTips'

When writing a new pack with a bitmap, it is sometimes convenient to
indicate some reference prefixes which should receive priority when
selecting which commits to receive bitmaps.

A truly motivated caller could accomplish this by setting
'pack.islandCore', (since all commits in the core island are similarly
marked as preferred) but this requires callers to opt into using delta
islands, which they may or may not want to do.

Introduce a new multi-valued configuration, 'pack.preferBitmapTips' to
allow callers to specify a list of reference prefixes. All references
which have a prefix contained in 'pack.preferBitmapTips' will mark their
tips as "preferred" in the same way as commits are marked as preferred
for selection by 'pack.islandCore'.

The choice of the verb "prefer" is intentional: marking the NEEDS_BITMAP
flag on an object does *not* guarantee that that object will receive a
bitmap. It merely guarantees that that commit will receive a bitmap over
any *other* commit in the same window by bitmap_writer_select_commits().

The test this patch adds reflects this quirk, too. It only tests that
a commit (which didn't receive bitmaps by default) is selected for
bitmaps after changing the value of 'pack.preferBitmapTips' to include
it. Other commits may lose their bitmaps as a byproduct of how the
selection process works (bitmap_writer_select_commits() ignores the
remainder of a window after seeing a commit with the NEEDS_BITMAP flag).

This configuration will aide in selecting important references for
multi-pack bitmaps, since they do not respect the same pack.islandCore
configuration. (They could, but doing so may be confusing, since it is
packs--not bitmaps--which are influenced by the delta-islands
configuration).

In a fork network repository (one which lists all forks of a given
repository as remotes), for example, it is useful to set
pack.preferBitmapTips to 'refs/remotes/<root>/heads' and
'refs/remotes/<root>/tags', where '<root>' is an opaque identifier
referring to the repository which is at the base of the fork chain.

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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