It comes handy to update more than one traking directory at the same
time.
Extend the "tracking_dir" command line parameter to accept one or more
parameters, that will all be used as the original "tracking_dir".

Test: build/synchronize_owners common-mainline android-mainline \
  common11-5.4 common12-5.10 common12-5.4 common-4.19-stable
Signed-off-by: Alessio Balsini <balsini@google.com>
Change-Id: Ibdf84a70c52acb3e3a574b832ea023192c5c0481