graphlog: remove too early return from getgraphlogrevs() for empty repo

Even if repository is empty, null revision should exist.

mercurial/cmdutil.py

@@ -1830,8 +1830,6 b' def getgraphlogrevs(repo, pats, opts):'
     callable taking a revision number and returning a match objects
     filtering the files to be detailed when displaying the revision.
     """
-    if not len(repo):
-        return [], None, None
     limit = loglimit(opts)
     # Default --rev value depends on --follow but --follow behaviour
     # depends on revisions resolved from --rev...

tests/test-glog.t

@@ -1541,6 +1541,9 b' have 2 filelog topological heads in a li'
   $ testlog --follow
   []
   []
+  $ testlog -rnull
+  ['null']
+  []
   $ echo a > a
   $ echo aa > aa
   $ echo f > f