crecord: add an experimental option for space key to move cursor down

I really want to have an option of toggling a selection on a line and also
moving cursor down as a single keystroke. It also kinda makes sense for space
key to do this, because some other curses UIs in the wild do this (e.g. various
file managers, htop). So I got an idea to make a config option that defaults to
False for compatibility, but allows making crecord UI a lot more useful for
people with big hunks.

We add this an experimental option to experiment with this behavior.

mercurial/crecord.py

@@ -1588,6 +1588,8 b' are you sure you want to review/edit and'
             return True
         elif keypressed in [' '] or (test and keypressed in ["TOGGLE"]):
             self.toggleapply()
+            if self.ui.configbool('experimental', 'spacemovesdown'):
+                self.downarrowevent()
         elif keypressed in ['A']:
             self.toggleall()
         elif keypressed in ['e']: