utils/github: handle non-standard tap remotes

[Rylan12]

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Fixes #10881

This PR adds a Tap#remote_repo method to extract the remote repository name from the tap remote url. This should be used instead of Tap#full_name for methods in the Utils::GitHub module so that those methods can be used on taps that use non-standard urls.

[BrewTestBot]

Review period will end on 2021-03-23 at 00:00:00 UTC.

[MikeMcQuaid]

Nice work!

[BrewTestBot]

Review period ended.