dgit: Move canonicalise_suite into fetch()

dgit: Move canonicalise_suite into fetch()

fetchpullargs has to run before we have dealt with multisuites, which
cannot be straightforwardly canonicalised.

Move the canonicalisation out of fetchpullargs.  Previously,
fetchpullargs would always call canonicalise_suite if it wasn't going
to fail.  In one of the cases it would print a message, but we have
just changed canonicalise_suite to print a message too, so the one
from fetchpullargs was redundant.

fetchpullargs is called right before fetch, in cmd_fetch; and, in
cmd_pull.  In cmd_pull the split brain check intervenes between
fetchpullargs and pull which calls fetch.  So overall there is now no
functional change other than the loss of the now-redundant message.

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