Send back status output in FetchRemote

In https://gitlab.com/gitlab-org/gitlab/-/issues/351340#note_834483113, we noticed that FetchRemote returns a tags_changed output, but it doesn't tell us which tags were changed, or whether they were updated or added. This would be helpful to track down what happened during a project import.

The scanner currently parses each line into FetchStatusLine in https://gitlab.com/gitlab-org/gitaly/blob/da12f147d256dadc5da02ef039aa83c6e13ab2ac/internal/git/fetch_scanner.go#L25-42.

I think we should optionally return a structure holding this information. Alternatively, sending back the raw text might be enough as well.

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