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.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).