Extract config.get_mailer(...) to make it available to hook writers

Currently, wrapper Pytho scripts like the example post-receive which call
git_multimail.run_as_* with a mailer argument have to hardcode the
mailer, or cut-and-paste a not-so-trivial piece of code from
git_multimail.py. Extract this piece of code to make it available to the
wrappers.
multimailhook.dontComputeNewRevs: disable new commits detection on demand

This is useful as a workaround when using the script as an update hook.

I use git-multimail to track changes to origin/master after a fetch, and
the new commits detection goes wrong most of the time because the fetch
updated both origin/master and origin/HEAD, and the hook is ran after the
actual update. In this use-case, the new commit detection is useless and
counter-productive.
normalize changed ref names in computation of exclude revspec

When the user calls git-multimail manually, the refname may be given as a
short name (e.g. master), but the comparison done a few lines after the
change in _compute_other_ref_sha1s() to exclude refs that are part of the
current change is done on the full name (e.g. refs/heads/master).
Normalize the ref names before the comparison to fix this.