revset: make follow() accept empty startrev

This is the same behavior as ancestors(emptyset).

mercurial/revset.py

@@ -913,9 +913,6 b' def _follow(repo, subset, x, name, follo'
     revs = None
     if 'startrev' in args:
         revs = getset(repo, fullreposet(repo), args['startrev'])
-        if not revs:
-            raise error.RepoLookupError(
-                _("%s expected at least one starting revision") % name)
     if 'file' in args:
         x = getstring(args['file'], _("%s expected a pattern") % name)
         if revs is None:

tests/test-log.t

@@ -742,8 +742,6 b' follow files starting from multiple revi'
 follow files starting from empty revision:
 
   $ hg log -T '{rev}: {files}\n' -r "follow('glob:*', startrev=.-.)"
-  abort: follow expected at least one starting revision!
-  [255]
 
 follow starting from revisions: