sequencer: drop repository argument from run_git_commit()

sequencer: drop repository argument from run_git_commit()

When we switched to using an external git-commit call in b0a3186140
(sequencer: simplify root commit creation, 2019-08-19), this function
didn't need to care about the repository object any more.

Arguably we could be passing along the repository path to the external
git-commit by using "--git-dir=r->path" here. But for the most part the
sequencer code relies on sub-process finding the same repository we're
already in (using the same environment variables or discovery process we
did). But we don't have a convenient interface for doing so, and there's
no indication that we need to. Let's just drop the unused parameter for
now.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>