crecord: redraw the screen when starting up chunkselector

Failure to do this can cause screen corruption like:

    <headerline>
     [X] filename.cc
    <several blank lines>
    <output from previous iteration of split that happened to be here>

I believe this might only happen in some terminals, and maybe only when using
the "alternate screen". Regardless of the exact conditions to reproduce, it
should be safe to always clear it when starting up and is probably the correct
thing to do anyway :)

Differential Revision: https://phab.mercurial-scm.org/D6131

mercurial/crecord.py

@@ -1764,6 +1764,12 b' are you sure you want to review/edit and'
         except curses.error:
             self.usecolor = False
 
+        # In some situations we may have some cruft left on the "alternate
+        # screen" from another program (or previous iterations of ourself), and
+        # we won't clear it if the scroll region is small enough to comfortably
+        # fit on the terminal.
+        self.stdscr.clear()
+
         # available colors: black, blue, cyan, green, magenta, white, yellow
         # init_pair(color_id, foreground_color, background_color)
         self.initcolorpair(None, None, name="normal")