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

mercurial/hg.py

@@ -711,9 +711,9 b' def clone(ui, peeropts, source, dest=Non'
                 uprev = None
                 status = None
                 if checkout is not None:
-                    try:
-                        uprev = destrepo.lookup(checkout)
-                    except error.RepoLookupError:
+                    if checkout in destrepo:
+                        uprev = checkout
+                    else:
                         if update is not True:
                             try:
                                 uprev = destrepo.lookup(update)