Fix the TortoiseHg Windows registry query for UE 4.3

Epic have done away with FPlatformMisc::GetRegistryString() in favour
of FPlatformMisc::GetStoredValue(), but the new function doesn't provide
a way to query HKEY_LOCAL_MACHINE, so I've had to use the lower level
FPlatformMisc::QueryRegKey() instead.