tf-sync-repos.sh: force update to GitHub

1 Fixed an error when revising a tag.
    The tag had already been pushed to GitHub,
    so we couldn't push the revised tag without force.
    Solved the issue by using the '--force' option.
2. Increase the num-to-keep to 30

Signed-off-by: Arthur She <arthur.she@linaro.org>
Change-Id: I358517c97167a7e1ffc8521de3cfbc723ce26fa3
2 files changed