docs: say "commits" in the --depth option wording for git-clone

docs: say "commits" in the --depth option wording for git-clone

It is not wrong to talk about "revisions" here, but in this context
revisions are always commits, and that is how we already name it in the
git-fetch docs. So align the docs by always referring to "commits".

Signed-off-by: Junio C Hamano <gitster@pobox.com>