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).
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state