git-debrebase: Set $debugcmd_when_debuglevel = 2

This produces much better (less noisy) debug output.  git-debrebase
calls subcommands an awful lot and they are generally very
uninteresting.

If this now turns out to be insufficient, we will add additional calls
to printdebug.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>