When `keep_divergent_refs` is enabled, provide users feedback about what didn't get pushed

Follow-up from !1915 (merged):

One important thing to note here: There is no feedback to the user that some of their refs may not have been selected for mirroring due to a divergence. This can result in a situation where some branches weren't pushed at all, and so no error message is ever surfaced, making it look like everything is up-to-date.

As this functionality is behind a feature flag, Jacob considered this an acceptable trade-off for the moment. As we test and expand on the feature, we can consider things like adding a field to the response object to have more granular feedback.