This patch adds a new --rev flag to hg branch which can be used to change branch
of revisions. This is motivated from topic extension where you can change topic
on revisions but this one has few restrictions which are:
1) You cannot change branch name in between the stack
2) You cannot change branch name and set it to an existing name
3) You cannot change branch of non-linear set of commits
Tests are added for the same.
An experimental flag `--rev` to `hg branch` which can be used to change
branch of changesets.