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.