ui: edit(): rebase, graft: set HGREVISION environment variable for an editor rebase and graft commands set 'rebase_source' or 'source' extras for the revision. Allow an editor to access the extras using HGREVISION environment variable. This may be useful when an editor is actually a script which modifies a commit message. The name 'HGREVISION' has been selected as transplant already sets this variable for its filters (--filter).