For ExtensionDistributor to provide git metadata, we need to be able to
store the cache file inside the extension directory itself. The GitInfo
class will now first check if the $wgGitInfoCacheDirectory is populated,
otherwise it will fallback to "$extensionDir/gitinfo.json".
Bug: T122769
Change-Id: Ib3457589ca6899925ae4610cfcdae22af8eaaaeb
