[PATCH 3/6] Make showdate use "Linus format"

This makes showdate use the same date format as
git-rev-list --pretty, and gives some speedup. It might also
be more portable.

Note that this changes the calling convention: the previous
version used seconds from $1, but timezone from the global
variable $date. cg-mkpatch is modified to the new way.

Also fixes bash's belief that number literals starting with
zero are octal.

Signed-off-by: Dan Holmsand <redacted>
---