progress bars for commit a bit noisy in non-interactive mode

[Martin Pool]

Our progress bars detect if they're being run from a terminal or not.
(The exact condition, I think, depends on not just whether there's a
tty present but also whether we expect it can rewrite lines and so
on.)

Cases where this can happen include:
 * running from cron or some kind of auto-builder
 * running as a subcommand of vim, emacs, or another editor - and
there are several variations here depending on whether it's vim or
gvim, and whether under a plain or tty subprocess of emacs
 * running as input to less(?)

It looks like the recent changes to commit make it show more progress
phases; as a consequence it writes a lot more output even when
committing a single file change.  This is invisible when run in a
regular terminal but looks really noisy in gvim - I get a screenful of
output.

To make a long story short I propose we just suppress progress bars
entirely in non-interactive mode.  When there is no one watching
Bazaar run (eg cron), this is a clear win.  If someone is watching
Bazaar run (eg commit under vim) they get less progress feedback, but
what we give now is not very useful.

-- 
Martin