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.

amend: query the wdir parent after taking lock (issue5266) If we query wdir parent without taking a lock, that can lead to bugs because the wdir parent can change is another process has changed the wdir parent. One such example of this was issue 5266. When a user is running amend and that amend is waiting for commit message, the user runs another amend which waits for lock. The second amend is waiting for lock, but has already read the description from the parent of working directory to use. Once the first amend completes the wdir parent changes but we still have the description from an old wdir parent. This patch fixes the bug by querying the description after taking lock. Attempts were made to add test for this but the results were unstable because they depend on time in which lock is released.