ui: remove unreachable branches and function calls from write() (issue6059)
This is at least faster than ui.write() of 4.8.2.
$ HGRCPATH=/dev/null hg files -R mozilla-central --time >/dev/null
4.8.2: time: real 2.340 secs (user 2.310+0.000 sys 0.020+0.000)
4.9rc0: time: real 2.580 secs (user 2.550+0.000 sys 0.020+0.000)
this: time: real 2.230 secs (user 2.210+0.000 sys 0.020+0.000)
Maybe the formatter should own a resolved write() function because it will
just call dest.write(msg) most of the time, but that would be too much for
stable.