prune: save reachable-from-recent objects with bitmaps

We pass our prune expiration to mark_reachable_objects(), which will
traverse not only the reachable objects, but consider any recent ones as
tips for reachability; see d3038d22f9 (prune: keep objects reachable
from recent objects, 2014-10-15) for details.

However, this interacts badly with the bitmap code path added in
fde67d6896 (prune: use bitmaps for reachability traversal, 2019-02-13).
If we hit the bitmap-optimized path, we return immediately to avoid the
regular traversal, accidentally skipping the "also traverse recent"
code.

Instead, we should do an if-else for the bitmap versus regular
traversal, and then follow up with the "recent" traversal in either
case. This reuses the "rev_info" for a bitmap and then a regular
traversal, but that should work OK (the bitmap code clears the pending
array in the usual way, just like a regular traversal would).

Note that I dropped the comment above the regular traversal here.  It
has little explanatory value, and makes the if-else logic much harder to
read.

Here are a few variants that I rejected:

  - it seems like both the reachability and recent traversals could be
    done in a single traversal. This was rejected by d3038d22f9 (prune:
    keep objects reachable from recent objects, 2014-10-15), though the
    balance may be different when using bitmaps. However, there's a
    subtle correctness issue, too: we use revs->ignore_missing_links for
    the recent traversal, but not the reachability one.

  - we could try using bitmaps for the recent traversal, too, which
    could possibly improve performance. But it would require some fixes
    in the bitmap code, which uses ignore_missing_links for its own
    purposes. Plus it would probably not help all that much in practice.
    We use the reachable tips to generate bitmaps, so those objects are
    likely not covered by bitmaps (unless they just became unreachable).
    And in general, we expect the set of unreachable objects to be much
    smaller anyway, so there's less to gain.

The test in t5304 detects the bug and confirms the fix.

I also beefed up the tests in t6501, which covers the mtime-checking
code more thoroughly, to handle the bitmap case (in addition to just
"loose" and "packed" cases). Interestingly, this test doesn't actually
detect the bug, because it is running "git gc", and not "prune"
directly. And "gc" will call "repack" first, which does not suffer the
same bug. So the old-but-reachable-from-recent objects get scooped up
into the new pack along with the actually-recent objects, which gives
both a recent mtime. But it seemed prudent to get more coverage of the
bitmap case for related code.

Reported-by: David Emett <dave@sp4m.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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