combinator should update bincache in the active environment upon chbranch

[Glyph Lefkowitz]

If I already have combinator_paths/bincache on my $PATH, it's inconvenient to have to start another shell (and hence run environment.py) every time I chbranch a new project. It should automatically regenerate binary links immediately. (Perhaps even pruning, if I de-branch?)

[Tristan Seligmann (mithrandi)]

This sounds like a great idea, especially since I don't run environment.py from my shell startup scripts (since it takes waaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaay too long seriously how does anyone ever do this without going insane) so it only ever gets run manually.