Fix slow tag ancestors

[C85297]

In #437 I added the match_tag_ancestors feature to return commits that were ancestors of matching tags.

That initial implementation turned out to be poorly optimised, and in large repos with many tags and many commits, a nested loop over each commit and each tag becomes very slow.

This pull request optimises the match_tag_ancestors feature:

1. Rather than resolving the commit each tag targets inside each loop iteration, each tag is resolved to its commit once, before the loop.
2. Check for commits that match one of the already resolved tag targets and instantly return
3. For all other commits, use git tag --contains to get a list of tags the commit is an ancestor of, and check if any of those tags match the eligible tags we're looking for.

My testing showed around 100x less subprocesses spawned and 40x faster check runs.

[linux-foundation-easycla]

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[C85297]

It seems EasyCLA will require me to get a corporate CLA approval completed, this may take a while :/