Allow configurable ObjectCheckers in fetch

RecievePack already honors fsck settings for safeForWindows and
safeForMacOS.  Allow those same checks to be performed during fetch
through a caller-configurable ObjectChecker.

Default the fetch fsck options to match the current platform, as
it can be reasonably assumed the repository will be accessed here.

Change-Id: I3c0f411fad209c6bd8fb9c4acf5c55a6799a6a2a