[patch] limit log to a single branch

[Alexis S. L. Carvalho]

Thus spake Steve Borho:
> TortoiseHg has a 'branch filter' for it's changelog grapher.  It starts
> at the branch tip and walks backwards only showing changesets explicitly
> marked as part of the branch their ancestors.
> 
> Would people be interested in me back-porting this to the graphlog
> extension?

How do you draw non-contiguous branches?  E.g. (showing only branch
names; time goes from bottom to top):

foo     foo
  \     /
   \   /
    bar
   /   \
  /     \
foo    foo

Arguably, even if "bar" is in a different branch, you'll want to show
it (because it's a merge, not because it's a branching point).

Alexis