S512535 Enable megarepo extension for watchman when we disable autopull

Summary:
We disabled this because having megarepo enabled would override autopull behavior, and pulling while in a rebase would lead to a deadlock.
As of D73681487 we have decoupled megarepo from autopull, so we're enabling megarepo again which will allow us to short circuit and read translated commits if they are present in the local repo from a previous fetch.

Reviewed By: jdelliot

Differential Revision: D73684292

fbshipit-source-id: 194dd24d1a4160ed21114d4cb2ae10e31d9d778a

Author: Péter Takács

watchman/scm/Mercurial.cpp

@@ -180,11 +180,7 @@ w_string Mercurial::mergeBaseWith(
                    "-r",
                    revset,
                    "--config",
-                   "ui.autopullcommits=false",
-                   "--config",
-                   "extensions.megarepo=!"}, // disable "megarepo" extension
-                                             // because it does not respect the
-                                             // ui.autopullcommits flag
+                   "ui.autopullcommits=false"},
                   makeHgOptions(requestId),
                   "query for the merge base");
             }