editor: prefer 'intermediate-source' extra to use for HGREVISION environment variable

Revision 51930a7180bd ('graft: record intermediate grafts in extras') introduced
'intermediate-source' extra which refers to the closest graft source.

As 'intermediate-source' extra provides more detailed information about the source
changeset than 'source' one, it is better to prefer the first one to use as a
value of HGREVISION environment variable for an editor.

mercurial/ui.py

@@ -809,7 +809,7 b' class ui(object):'
             environ = {'HGUSER': user}
             if 'transplant_source' in extra:
                 environ.update({'HGREVISION': hex(extra['transplant_source'])})
-            for label in ('source', 'rebase_source'):
+            for label in ('intermediate-source', 'source', 'rebase_source'):
                 if label in extra:
                     environ.update({'HGREVISION': extra[label]})
                     break