Allow special team names.

When using the `team:` selector, allow to use some special "team names"
to get stats about groups of users that do not actually correspond to a
GitHub team.

Special team names allowed are:

* `__contributors`: All users who contributed to the repository
* `__committers`: All users who committed to the repository
* `__commenters`: All users who opened a ticket/PR, or who commented on
  one
* `__collaborators`: All users registered as collaborators on the
  repository

incenp/grainyhead/metrics.py

@@ -182,11 +182,7 @@ def _get_parser(self):
             ).leaveWhitespace()
             team_filter = (
                 (pp.Literal('team:') + filter_value)
-                .set_parse_action(
-                    lambda t: TeamFilter(
-                        t[1], [m.login for m in self._repo.get_team(t[1])]
-                    )
-                )
+                .set_parse_action(self._team_action)
                 .leave_whitespace()
             )
             user_filter = (
@@ -248,6 +244,20 @@ def _expression_action(self, tokens):
         else:
             return tokens[0]
 
+    def _team_action(self, tokens):
+        team_name = tokens[1]
+        if team_name == '__contributors':
+            users = self._repo.contributors
+        elif team_name == '__committers':
+            users = self._repo.committers
+        elif team_name == '__commenters':
+            users = self._repo.commenters
+        elif team_name == '__collaborators':
+            users = self._repo.collaborators
+        else:
+            users = [u.login for u in self._repo.get_team(team_name)]
+        return TeamFilter(team_name, users)
+
 
 class MetricsFormatter(object):
     """Write a MetricsReportSet object."""