commit-graph: don't consider "replace" objects with "verify"

commit-graph: don't consider "replace" objects with "verify"

Extend the code added in d6538246d3d (commit-graph: not compatible
with replace objects, 2018-08-20) which ignored replace objects in the
"write" command to ignore it in the "verify" command too.

We can just move this assignment to the cmd_commit_graph(), it
dispatches to "write" and "verify", and we're unlikely to ever get a
sub-command that would like to consider replace refs.

This will make tests added in eddc1f556cd (mktag tests: test
update-ref and reachable fsck, 2021-06-17) pass in combination with
the "GIT_TEST_COMMIT_GRAPH" mode added in 859fdc0c3cf (commit-graph:
define GIT_TEST_COMMIT_GRAPH, 2018-08-29), except that mode is
currently broken (but is being fixed concurrently). See the discussion
starting at [1].

1. https://lore.kernel.org/git/87wnmihswp.fsf@evledraar.gmail.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>