shallow: handling fetch relative-deepen

shallow: handling fetch relative-deepen

When a shallowed repository gets deepened beyond the beginning of a
merged branch, we may end up with some shallows that are hidden behind
the reachable shallow commits. Added test 'fetching deepen beyond
merged branch' exposes that behaviour.

An example showing the problem based on added test:
0. Whole initial git repo to be cloned from
   Graph:
   *   033585d (HEAD -> main) Merge branch 'branch'
   |\
   | * 984f8b1 (branch) five
   | * ecb578a four
   |/
   * 0cb5d20 three
   * 2b4e70d two
   * 61ba98b one

1. Initial shallow clone --depth=3 (all good)
   Shallows:
   2b4e70da2a10e1d3231a0ae2df396024735601f1
   ecb578a3cf37198d122ae5df7efed9abaca17144
   Graph:
   *   033585d (HEAD -> main) Merge branch 'branch'
   |\
   | * 984f8b1 five
   | * ecb578a (grafted) four
   * 0cb5d20 three
   * 2b4e70d (grafted) two

2. Deepen shallow clone with fetch --deepen=1 (NOT OK)
   Shallows:
   0cb5d204f4ef96ed241feb0f2088c9f4794ba758
   61ba98be443fd51c542eb66585a1f6d7e15fcdae
   Graph:
   *   033585d (HEAD -> main) Merge branch 'branch'
   |\
   | * 984f8b1 five
   | * ecb578a four
   |/
   * 0cb5d20 (grafted) three
   ---
   Note that second shallow commit 61ba98be443fd51c542eb66585a1f6d7e15fcdae
   is not reachable.

On the other hand, it seems that equivalent absolute depth driven
fetches result in all the correct shallows. That led to this proposal,
which unifies absolute and relative deepening in a way that the same
get_shallow_commits() call is used in both cases. The difference is
only that depth is adapted for relative deepening by measuring
equivalent depth of current local shallow commits in the current remote
repo. Thus a new function get_shallows_depth() has been added and the
function get_reachable_list() became redundant / removed.

Same example showing the corrected second step:
2. Deepen shallow clone with fetch --deepen=1 (all good)
   Shallow:
   61ba98be443fd51c542eb66585a1f6d7e15fcdae
   Graph:
   *   033585d (HEAD -> main) Merge branch 'branch'
   |\
   | * 984f8b1 five
   | * ecb578a four
   |/
   * 0cb5d20 three
   * 2b4e70d two
   * 61ba98b (grafted) one

The get_shallows_depth() function also shares the logic of the
get_shallow_commits() function, but it focuses on counting depth of
each existing shallow commit. The minimum result is stored as
'data->deepen_relative', which is set not to be zero for relative
deepening anyway. That way we can always sum 'data->deepen_relative'
and 'depth' values, because 'data->deepen_relative' is always 0 in
absolute deepening.
To avoid duplicating logic between get_shallows_depth() and
get_shallow_commits(), get_shallow_commits() was modified so that
it is used by get_shallows_depth().

Signed-off-by: Samo Pogačnik <samo_pogacnik@t-2.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>