commit-reach: use `size_t` to track indices when computing merge bases

commit-reach: use `size_t` to track indices when computing merge bases

The functions `repo_get_merge_bases_many()` and friends accepts an array
of commits as well as a parameter that indicates how large that array
is. This parameter is using a signed integer, which leads to a couple of
warnings with -Wsign-compare.

Refactor the code to use `size_t` to track indices instead and adapt
callers accordingly. While most callers are trivial, there are two
callers that require a bit more scrutiny:

  - builtin/merge-base.c:show_merge_base() subtracts `1` from the
    `rev_nr` before calling `repo_get_merge_bases_many_dirty()`, so if
    the variable was `0` it would wrap. This code is fine though because
    its only caller will execute that code only when `argc >= 2`, and it
    follows that `rev_nr >= 2`, as well.

  - bisect.ccheck_merge_bases() similarly subtracts `1` from `rev_nr`.
    Again, there is only a single caller that populates `rev_nr` with
    `good_revs.nr`. And because a bisection always requires at least one
    good revision it follws that `rev_nr >= 1`.

Mark the file as -Wsign-compare-clean.

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