Make goal listing use only the first sentence of the description.

Now that the description can be inferred from a task docstring,
this is pertinent.

Testing Done:
Manual testing before and after. Ran the list_goals tests.

Bugs closed: 1682

Reviewed at https://rbcommons.com/s/twitter/r/2366/

src/python/pants/backend/core/tasks/list_goals.py

@@ -25,8 +25,10 @@ def report():
       undocumented = []
       max_width = 0
       for goal in Goal.all():
-        if goal.description:
-          documented_rows.append((goal.name, goal.description))
+        desc = goal.description
+        if desc:
+          first_sentence = desc.partition('\n')[0]
+          documented_rows.append((goal.name, first_sentence))
           max_width = max(max_width, len(goal.name))
         elif self.get_options().all:
           undocumented.append(goal.name)