This makes diff archives to be reproducible.
The value is expected to be passed from CLI applications via the $SOUCE_DATE_EPOCH env var.
See https://reproducible-builds.org/docs/source-date-epoch/
for the $SOURCE_DATE_EPOCH specification.
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>