revset: make follow() reject more than one start revisions

Taking only the last revision is inconsistent because ancestors(set) follows
all revisions given, and theoretically follow(startrev=set) == ancestors(set).
I'm planning to add a support for multiple start revisions, but that won't fit
to the 4.0 time frame. So reject multiple revisions now to avoid future BC.

len(revs) might be slow if revs were large, but we don't care since a valid
revs should have only one element.

mercurial/revset.py

@@ -1027,10 +1027,11 b' def _follow(repo, subset, x, name, follo'
         x = getstring(l[0], _("%s expected a pattern") % name)
         rev = None
         if len(l) >= 2:
-            rev = getset(repo, fullreposet(repo), l[1]).last()
-            if rev is None:
+            revs = getset(repo, fullreposet(repo), l[1])
+            if len(revs) != 1:
                 raise error.RepoLookupError(
-                        _("%s: starting revision set cannot be empty") % name)
+                        _("%s expected one starting revision") % name)
+            rev = revs.last()
             c = repo[rev]
         matcher = matchmod.match(repo.root, repo.getcwd(), [x],
                                  ctx=repo[rev], default='path')