add --no-current-branch option to invert --current-branch

For most commands, this is more about providing a way to undo earlier
command line options (e.g. `repo info -c --no-current-branch`) which
can be helpful for scripting & automation.  But for the sync command,
this is helpful to undo the setting that exists in the manifest itself.

With this in place, tweak the sync current_branch_only logic to only
apply the manifest settings when the user hasn't specified a command
line option.

Bug: https://crbug.com/gerrit/12401
Change-Id: I21e2384624680cc740d1b5d1e49c50589d2fe6a0
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/304903
Reviewed-by: Raman Tenneti <rtenneti@google.com>
Tested-by: Mike Frysinger <vapier@google.com>