Add support for fetching tags

[SludgePhD]

This adds a fetch-tags option, intended to be compatible with actions/checkout.

If true, --no-tags is replaced with --tags in the git fetch command line.

I've successfully tested the functionality in one of my repos.

[SludgePhD]

It looks like there's a problem with the recent changes to ci/test.sh in 6ad5001, causing the CI failures. The grep invocation rejects workflow commits created for PRs (which are seemingly always merge commits).