pack-bitmap: parse commits in `find_pseudo_merge_group_for_ref()`

pack-bitmap: parse commits in `find_pseudo_merge_group_for_ref()`

`find_pseudo_merge_group_for_ref()` uses the commit's date to classify
it as either "stable" (older than the stable threshold) or "unstable"
(otherwise).

However, to find the relevant commit from a given OID, the function
`find_pseudo_merge_group_for_ref()` uses `lookup_commit()` which does
not parse commits.

Because an unparsed commit has its "date" set to zero, every candidate
is placed in the "stable" bucket regardless of its actual committer
timestamp. This means the `bitmapPseudoMerge.*.threshold` and
`stableThreshold` configuration options have no effect: the
stable/unstable split is always determined by comparing against zero
rather than the real commit date.

The net result is that pseudo-merge groups are partitioned by
`stableSize` instead of the intended decay-based sizing, and the
`sampleRate` knob (which only applies to the unstable path) is never
exercised.

Fix this by calling `repo_parse_commit()` after `lookup_commit()`,
bailing out of the callback if parsing fails.

The corresponding test configures two pseudo-merge groups that both
match all tags. The "stable" group uses `threshold=1.month.ago`, and the
"all" group uses `threshold=now`. The test use our custom
"GIT_TEST_DATE_NOW" environment variable by setting it to the value of
"$test_tick" to align Git's notion of "now" (and therefore
"1.month.ago") with the `test_tick` timestamps, so the commits appear to
be younger than one month: only the "all" group matches them, producing
exactly one pseudo-merge.

Without the fix every commit has `date == 0`, which satisfies `date <=
threshold` for both groups (since 0 is older than one month ago), and
the "stable" group erroneously matches as well.

Now that commits are correctly classified as "unstable", the bug
described in the test exercising the "sampleRate=0" test is reachable,
and the test is marked as failing. It will be fixed in a following
commit.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>