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).
(binary:application/octet-stream)