devel/got: update to 0.91

User-visible changes:
- add merge -M option which tells 'got merge' not to fast-forward a reference
- add initial implementation of 'gotadmin dump' which creates Git bundle files
- add initial implementation of 'gotadmin load' which loads Git bundle files
- abort histedit if the user quits the editor without saving the script
- add support for keywords as <commit> arguments to got and tog