clone: don't fail with --update for non-local clones (issue3578)

This was broken by 5884812686f7 due to lack of test coverage. This
adds a test and fixes the defect.

mercurial/hg.py

@@ -398,7 +398,7 b' def clone(ui, peeropts, source, dest=Non'
 
             if update:
                 if update is not True:
-                    checkout = srcrepo.lookup(update)
+                    checkout = srcpeer.lookup(update)
                 for test in (checkout, 'default', 'tip'):
                     if test is None:
                         continue

tests/test-http.t

@@ -77,6 +77,24 b' clone via pull'
   adding bar
   $ cd ..
 
+clone over http with --update
+
+  $ hg clone http://localhost:$HGPORT1/ updated --update 0
+  requesting all changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 2 changesets with 5 changes to 5 files
+  updating to branch default
+  4 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg log -r . -R updated
+  changeset:   0:8b6053c928fe
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     1
+  
+  $ rm -rf updated
+
 incoming via HTTP
 
   $ hg clone http://localhost:$HGPORT1/ --rev 0 partial