have hg_import_helper always return true from ui.plain()

Summary:
Update the HgUI object used by hg_import_helper to always return true from the
`plain()` function, regardless of whether HGPLAIN is set in the environment or
not.

This should help ensure that this script is never affected by user-defined
configuration settings.  This should also help ensure that mercurial won't try
to print progress bars or other strange things.

Reviewed By: quark-zju

Differential Revision: D13008639

fbshipit-source-id: afe581958470c4c4b89825a259c460ece4e20fe7

eden/fs/store/hg/hg_import_helper.py

@@ -185,6 +185,9 @@ class HgUI(mercurial.ui.ui):
         self.fout = sys.stderr
         self.ferr = sys.stderr
 
+    def plain(self, feature=None):
+        return True
+
     def interactive(self):
         return False