ui mode for extdiff

[TK Soh]

I created a command to be called by extdiff, but would to be able to
control the verbosity of it's output per hg's output modes, i.e.
<default>, verbose, debug & quiet. In my quick hack, I set the the
HG_UIMODE env var to one of ['default', 'verbose', 'debug', 'quiet'],
then has my custom command check and act according. I am also
wondering it this addition should not be limited to extdiff.

I wonder if anyone has any other suggestion on how to best implement
this, and whether such support will be good for inclusion for upcoming
mercurial release.