[linyiru]

With git_cache, a trusted job on one repository that fetched another private repository (a second checkout, a submodule) stored that pack under its own repository and default ref. A fork PR may read the default branch's caches, so it could restore that pack straight from cache-server with the git/ prefix, without the other repository's credentials.

Changes:

• Git cache keys now name their repository in clear: git/<owner>/<name>/<hash>. gitcache.KeyRepository parses them, and gitcache.EntryVersion is the git entry version.
• cache-server refuses to save a git entry whose key names a repository other than the job's own. actions/cache keys under any other version are untouched.
• Lookups skip any git entry that does not name the job's own repository. A fork PR therefore cannot restore entries saved in the old format either.
• README and changelog are updated. Images built before this change keep fetching from GitHub but no longer save to the git cache.

Tests:

• internal/cacheserver/gitentry_test.go covers which saves are accepted or refused, and that an old-format entry is not restored by a prefix lookup.
• internal/gitproxy/cache_test.go uses real keys and adds a cross-repository save test.
• internal/gitcache has a KeyRepository table.
• scripts/mutate.sh confirmed that both the save rule and the lookup filter are caught.

Review: a fresh reviewer agent reviewed the change. It confirmed that case, %2F, .. and redirects cannot get another repository saved under the job's scope. Its should-fix, that old entries were still readable by a direct client, is fixed here. Its test nits are also addressed.