extdatasource: use revsymbol() for converting to node

It's unclear what we want to support here. Perhaps it should just be
nodeids? It doesn't make much sense to share revnums between repos,
and bookmarks are probably too unstable. I used scmutil.revsymbol() to
preserve the current behavior. We can change later if we want to.

Differential Revision: https://phab.mercurial-scm.org/D3088

mercurial/scmutil.py

@@ -1134,7 +1134,7 b' def extdatasource(repo, source):'
 
             k = encoding.tolocal(k)
             try:
-                data[repo[k].rev()] = encoding.tolocal(v)
+                data[revsingle(repo, k).rev()] = encoding.tolocal(v)
             except (error.LookupError, error.RepoLookupError):
                 pass # we ignore data for nodes that don't exist locally
     finally: