t: add --no-tag option to test_commit

t: add --no-tag option to test_commit

One of the conveniences that test_commit offers is making a tag for each
commit. This makes it easy to refer to the commits in subsequent
commands. But it can also be a pain if you care about reachability,
because those tags keep the commits reachable even if they are rewound
from the branch they're made on.

The alternative is that scripts have to call test_tick, git-add, and
git-commit themselves. Let's add a --no-tag option to give them the
one-liner convenience of using test_commit.

This is in preparation for the next patch, which will add some more
calls. But I cleaned up an existing site to show off the feature. There
are probably more cleanups possible.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>