RefService.DeleteRefs: implement `except_with_prefix`

This is a follow-up to #49 (closed).

The except_with_prefix request argument for DeleteRefs is used only in post-import service of the Rails app, which is itself run for all imports except the "import by URL". That currently means only the "Manifest" import for Mercurial repos. See #49 (closed) for insights about this.

Edited Mar 19, 2021 by Georges Racinet

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