GitInfo: Allow cache to be in the extension directory itself

GitInfo: Allow cache to be in the extension directory itself

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