nmbug: Auto-checkout in clone if it wouldn't clobber

nmbug: Auto-checkout in clone if it wouldn't clobber

We currently auto-checkout after pull and merge to make those more
convenient.  They're guarded against data-loss with a leading
_insist_committed().  This commit adds the same convenience to clone,
since in most cases users will have no NMBPREFIX-prefixed tags in
their database when they clone.  Users that *do* have
NMBPREFIX-prefixed tags will get a warning (and I've bumped the
default log level to warning so folks who don't set --log-level will
see it) like:

  $ nmbug clone http://nmbug.notmuchmail.org/git/nmbug-tags.git
  Cloning into '/tmp/nmbug-clone.g9dvd0tv'...
  Checking connectivity: 16674, done.
  Branch config set up to track remote branch config from origin.
  Not checking out to avoid clobbering existing tags: notmuch::0.25, ...