dir: Clear summary cache when updating metadata

For example, if the url changes we need to re-read the summary. This
was causing issues in a repo test where we re-wrote the url, and then
the new summary was not downloaded because of a st_mtime based
If-Modified-After check in the python httpd.
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f