dgit: In code, rename splitbrain quilt modes to splitting

dgit: In code, rename splitbrain quilt modes to splitting

A "splitting" quilt mode is going to be one which requires split brain
mode.  But split brain mode is going to be possible in other quilt
modes too.

The existing name of the splitbrain quilt cache is correct: it is used
precisely for quilt fixup, in split brain modes (even for non
splitting quilt modes, now).  In split brain mode without quilt, it is
not needed or used.

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