reftable/stack: use geometric table compaction

reftable/stack: use geometric table compaction

To reduce the number of on-disk reftables, compaction is performed.
Contiguous tables with the same binary log value of size are grouped
into segments. The segment that has both the lowest binary log value and
contains more than one table is set as the starting point when
identifying the compaction segment.

Since segments containing a single table are not initially considered
for compaction, if the table appended to the list does not match the
previous table log value, no compaction occurs for the new table. It is
therefore possible for unbounded growth of the table list. This can be
demonstrated by repeating the following sequence:

git branch -f foo
git branch -d foo

Each operation results in a new table being written with no compaction
occurring until a separate operation produces a table matching the
previous table log value.

Instead, to avoid unbounded growth of the table list, the compaction
strategy is updated to ensure tables follow a geometric sequence after
each operation by individually evaluating each table in reverse index
order. This strategy results in a much simpler and more robust algorithm
compared to the previous one while also maintaining a minimal ordered
set of tables on-disk.

When creating 10 thousand references, the new strategy has no
performance impact:

Benchmark 1: update-ref: create refs sequentially (revision = HEAD~)
  Time (mean ± σ):     26.516 s ±  0.047 s    [User: 17.864 s, System: 8.491 s]
  Range (min … max):   26.447 s … 26.569 s    10 runs

Benchmark 2: update-ref: create refs sequentially (revision = HEAD)
  Time (mean ± σ):     26.417 s ±  0.028 s    [User: 17.738 s, System: 8.500 s]
  Range (min … max):   26.366 s … 26.444 s    10 runs

Summary
  update-ref: create refs sequentially (revision = HEAD) ran
    1.00 ± 0.00 times faster than update-ref: create refs sequentially (revision = HEAD~)

Some tests in `t0610-reftable-basics.sh` assert the on-disk state of
tables and are therefore updated to specify the correct new table count.
Since compaction is more aggressive in ensuring tables maintain a
geometric sequence, the expected table count is reduced in these tests.
In `reftable/stack_test.c` tests related to `sizes_to_segments()` are
removed because the function is no longer needed. Also, the
`test_suggest_compaction_segment()` test is updated to better showcase
and reflect the new geometric compaction behavior.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Acked-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>