when used without the --branch option, we clone with --no-checkout, to
avoid unnecessarily checking out files twice when the recorded sha1
doesn't happen to be the tip of the default branch. however, that would
leave the index in a dirty state which would make the subsequent
submodule update abort at some point, silently.

to deal with the problem, we ignore this type of index dirtyness and use
submodule update with --force.

Task-number: QTBUG-57289
Change-Id: I6fc9693b0eaadfb04d2d80f9c8f1f2e11be47ae9
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>