remote-modify: Clear cached summaries when the remote is modified.

This fixed some issues in the test where changing the url cause the
cache:ing to fail to update due to the mtimes being to new in combination
with the HTTP If-Modified-Since use.
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f