Fix describe tag issue when finding ancestor tag (#3376)

The runtime-versioner's audit subcommand fails when the current commit
is the tagged release commit due to a bug in `ancestor_tag` where it was
failing to trim the output from `git describe --tags`. I fixed this
earlier in #3369 since I was running into it there, but that hasn't been
reviewed/merged yet. This issue is causing problems for the latest
smithy-rs/SDK release, so I've pulled it out into a separate PR.

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