ui: add curses interface to tweakdefaults

This was part of the original proposal, and while *I* don't like the
curses interface, most users anecdotally seem to greatly prefer it to
plain text interfaces.

Differential Revision: https://phab.mercurial-scm.org/D1609

mercurial/ui.py

@@ -51,6 +51,8 b' tweakrc = """'
 rollback = False
 # Make `hg status` report copy information
 statuscopies = yes
+# Prefer curses UIs when available. Revert to plain-text with `text`.
+interface = curses
 
 [commands]
 # Make `hg status` emit cwd-relative paths by default.