Allow configuration of receive pack's ObjectChecker through fsck.*

Allow configuration of receive pack's ObjectChecker through fsck.*

fsck.allowLeadingZeroFileMode may be set true to permit pushing
broken trees with leading '0' in the file mode.

fsck.safeForWindows may be set true to require new trees to have
only file names that are safe on the Windows platform.

fsck.safeForMacOS may be set true to require new trees to have
only file names that do not cause collisions or confusion on the
Mac OS platform.

Change-Id: I1a225c1b3cd13c0d1a0d43fffe79355c501f49b7