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).
$ rm -rf a
$ hg init a
$ cd a
$ echo a > a
$ hg ci -Am0
adding a
$ hg tag t1 # 1
$ hg tag --remove t1 # 2
$ hg co 1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg tag -f -r0 t1
$ hg tags
tip 3:a49829c4fc11
t1 0:f7b1eb17ad24
$ cd ..