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).

mercurial/ui.py

@@ -722,10 +722,16 b' class ui(object):'
             f.write(text)
             f.close()
 
+            environ = {'HGUSER': user}
+            for label in ('source', 'rebase_source'):
+                if label in extra:
+                    environ.update({'HGREVISION': extra[label]})
+                    break
+
             editor = self.geteditor()
 
             util.system("%s \"%s\"" % (editor, name),
-                        environ={'HGUSER': user},
+                        environ=environ,
                         onerr=util.Abort, errprefix=_("edit failed"),
                         out=self.fout)