sync: fix deprecated command line option -f

In commit d9e5cf0e

 ("sync: invert --force-broken with --fail-fast") the
force-broken option has been deprecated. Accidentally the option has
been changed from Boolean to Value. This breaks all users of repo with:

  main.py: error: -f option requires an argument

This is easy to avoid by keeping the type.

Signed-off-by: Stefan Müller-Klieser <s.mueller-klieser@phytec.de>
Change-Id: Ia8b589cf41ac756d10c61e17ec8d76ba8f7031f9
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/235043
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Mike Frysinger <vapier@google.com>