bash_completion: add global support for -B|--bookmark Previously, only -r|--rev was parsed globally which meant 'hg push -B <tab>' would try to complete a path instead of a bookmark.

bash_completion: change --rev if-else block into a case Again, this doesn't change behavior but does make it easy to add cases in the next patch.

bash_completion: simplify if-else logic There is no change in functionality here but this makes refactoring this later easier.

bash_completion: add _hg_branches for list of branches