simplemerge: make context parameters non-optional

simplemerge: make context parameters non-optional

Also update the function docstring.

Differential Revision: https://phab.mercurial-scm.org/D382

--- a/mercurial/simplemerge.py	Thu Aug 24 21:30:51 2017 -0700
+++ b/mercurial/simplemerge.py	Thu Aug 24 21:30:51 2017 -0700
@@ -419,13 +419,10 @@
 
     return [name_a, name_b, name_base]
 
-def simplemerge(ui, localctx=None, basectx=None, otherctx=None, repo=None,
-                **opts):
+def simplemerge(ui, localctx, basectx, otherctx, repo=None, **opts):
     """Performs the simplemerge algorithm.
 
-    {local|base|other}ctx are optional. If passed, they (local/base/other) will
-    be read from and the merge result written to (local). You should pass
-    explicit labels in this mode since the default is to use the file paths.
+    The merged result is written into `localctx`.
     """
     def readctx(ctx):
         if not ctx: