clone: process 'lookup' return as an arbitrary symbol

In theory, checkout is expected to be a node here because it was returned by
peer.lookup.

In practice, multiple important extensions (like hg-git, hg-subversion) use
peers not backed by a mercurial repository where lookup cannot return a node.

Allowing arbitrary symbols is necessary to make these extensions working with
4.7.

We should probably introduce a new API in Core to have these extensions to
work without abusing the lookup API. In the meantime, a small change to
restore compatibility in 4.7 seems in order.

mercurial/hg.py

@@ -731,8 +731,14 b' def clone(ui, peeropts, source, dest=Non'
                 uprev = None
                 status = None
                 if checkout is not None:
-                    if checkout in destrepo:
+                    # Some extensions (at least hg-git and hg-subversion) have
+                    # a peer.lookup() implementation that returns a name instead
+                    # of a nodeid. We work around it here until we've figured
+                    # out a better solution.
+                    if len(checkout) == 20 and checkout in destrepo:
                         uprev = checkout
+                    elif scmutil.isrevsymbol(destrepo, checkout):
+                        uprev = scmutil.revsymbol(destrepo, checkout).node()
                     else:
                         if update is not True:
                             try: