Fix time offset calculation in case of unsigned time_t
Fix time offset calculation in case of unsigned time_t

Fix time offset calculation expression in case if time_t
is unsigned. This code works fine for signed and
unsigned time_t.

Signed-off-by: Mike Gorchak <7d1f4d7927ffce901b8d1a9a73999fde47cc115d@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git