main: add mercurial.main as an entry point

Summary:
The old mercurial.dispatch entry point works, except it breaks
extensions wrapping `dispatch.*` (ex. `dispatch.runcommand`).

That is because things happen in this order:
- Buck's python wrapper imports `mercurial.dispatch` as mod1.
- hgdemandimport is enabled.
- An extension imports `mercurial.dispatch` and it got mod2.
- mod2 != mod1. So any changes to mod2 won't be used.

Using a separate module (`mercurial.main`) as the entry point
solves the problem.

Reviewed By: DurhamG

Differential Revision: D6879873

fbshipit-source-id: 9779edac64bbeb53dd98dfab1b21575c3a60af01

mercurial/dispatch.py

@@ -5,7 +5,7 @@
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
-from __future__ import absolute_import, print_function
+from __future__ import absolute_import
 
 import difflib
 import errno
@@ -1010,8 +1010,3 @@ def rejectpush(ui, **kwargs):
     # mercurial hooks use unix process conventions for hook return values
     # so a truthy return means failure
     return True
-
-if __name__ == '__main__':
-    import hgdemandimport
-    hgdemandimport.enable()
-    run()

mercurial/main.py

@@ -0,0 +1,12 @@
+# Copyright 2018 Facebook, Inc.
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+
+from __future__ import absolute_import
+
+if __name__ == '__main__':
+    import hgdemandimport
+    hgdemandimport.enable()
+    from . import dispatch
+    dispatch.run()