zoekt-mirror-gerrit: Don't remove /a/ prefix from git clone URLs (#990)
The /a/ prefix is required for authenticated git operations in default Gerrit
configurations. Gerrit's HttpScheme intentionally includes /a/ in clone URLs
to trigger authentication. Removing it breaks instances using default config.
The ServerInfo API already returns the correct URL format based on the
instance's configuration, so we should use it as-is.
Amp-Thread-ID: https://ampcode.com/threads/T-6944ab20-837d-4e78-a9ad-51083263baec
Co-authored-by: Amp <amp@ampcode.com>