version/mkversion: don't break on tagged go.mod entries

I thought our versioning scheme would make go.mod include a commit hash
even on stable builds. I was wrong. Fortunately, the rest of this code
wants anything that 'git rev-parse' understands (to convert it into a full
git hash), and tags qualify.

Signed-off-by: David Anderson <danderson@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f