clone: avoid using repo.lookup() with binary nodeid
The code in hg.clone() is a bit of a mess, but it seems like the
"checkout" variable is always a binary nodeid (tests pass when run
with "assert len(checkout) == 20" before the
repo.lookup()). repo.lookup() will soon work only with string inputs,
so we need remove this use.
Differential Revision: https://phab.mercurial-scm.org/D3191
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).