evolve: store the replacements occurred during `hg evolve` at top level This patch starts storing the replacements made during the `hg evolve` command. The current dictionary is not used anywhere but will be used in future as we want to introduce abort functionality, so we want to delay writing of obsmarkers, storing these exchanges in statefile etc.

evolve: make the _solve* functions return a tuple This patch makes _solve* functions return a tuple which will contain information about whether we were able to solve the instability and if yes, then what was the newnode. For more details about tuple, look the fn docs added. There was some refactoring required in next command because of change in return values. This also leds to omitting the wdir update message in case of dry-run which is good.

evolvestate: rename the file to state.py and class name to cmdstate The evolvestate.py and the evolvestate class in it were introduced to help `hg evolve` grow continue and abort options. But the class and it's API can be easily used to store state information for other state files like we stored the information for grab state in the previous commit. Let's rename the the file and class.

branching: merge with stable

update: show "use 'hg evolve' to..." hint iff evolve command enabled Before this patch, one had to enable *all* commands (by including "allnewcommands" in the experimental.evolution config) for the hint to be shown.

obslog: drop period from summary line in accordance with convention

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.