date.c: switch to reentrant {gm,local}time_r

date.c: switch to reentrant {gm,local}time_r

Originally, git was intended to be single-thread executable.
`gmtime(3)' and `localtime(3)' can be used in such codebase
for cleaner code.

Overtime, we're employing multithread in our code base.

Let's phase out `gmtime(3)' and `localtime(3)' in favour of
`gmtime_r(3)' and `localtime_r(3)'.

Signed-off-by: Doan Tran Cong Danh <redacted>
Signed-off-by: Junio C Hamano <redacted>