clone: check update rev for being True

In 30be3aeb5344, there was an attempt to fallback to looking up the update
revision assuming it was always a rev but the documentation states:

  True means update to default rev, anything else is treated as a revision

Therefore, we should only fallback to looking up the update rev if it is not
True. This bug was found in hg-git and I couldn't think of a test that does
this in pure Mercurial since the source repository is checked for the revision
as well (and therefore gracefully falls back).

mercurial/hg.py

@@ -582,10 +582,11 b' def clone(ui, peeropts, source, dest=Non'
                     try:
                         uprev = destrepo.lookup(checkout)
                     except error.RepoLookupError:
-                        try:
-                            uprev = destrepo.lookup(update)
-                        except error.RepoLookupError:
-                            pass
+                        if update is not True:
+                            try:
+                                uprev = destrepo.lookup(update)
+                            except error.RepoLookupError:
+                                pass
                 if uprev is None:
                     try:
                         uprev = destrepo._bookmarks['@']