Fix race condition when storing GitConfiguration to disk

[DefaultRyan]

Summary of the pull request

The GitConfiguration class is thread-safe, but it still resulted in multiple threads trying to store the configuration file concurrently. This concurrent access mostly hit when browsing to a folder in File Explorer for the first time in the Git extension process life, as multiple threads tried to perform the initial Git detection concurrently. Making the _fileLock field static ensures that only one thread can access the file at a time.

Validation steps performed

Prior to fix, when using a debug build, multiple file IO exceptions were logged in the git integration console window when first browsing to a registered folder. After fix, these IO exceptions are no longer encountered.

PR checklist

• Closes Race condition storing git config (detects presence of command-line Git) #3685