grab: add a command to grab a commit and update to it
This patch adds anew command `hg grab` which will pick a commit and put it on
the top of the working directory parent and updates to it.
Earlier grab was an alias for `hg rebase -r <rev> -d .` which is now deleted to
make sure `hg grab` resolves to the grab command and that alias.
Continuing interrupted grab functionality is also there using the evolvestate
class.
Tests are also added for the new introduced command.