context: make changectx.mutable() benefit from .phase() logic This fixes "hg qimport -r null". Previous versions used to: - Traceback because null revision mutability was not defined - Add an empty -1.diff patch to the series The error message: abort: revision -1 is not mutable is symptomatic of a deeper problem in phase command revision handling. It could be fixed easily in the command itself but I feel a better fix must be done in phase API which raises the issue of phase updates atomicity: aborting in phases.advanceboundary/retractboundary requires a better rollback behaviour to avoid partial changes.