upload-pack: add deepen-since to cut shallow repos based on time

upload-pack: add deepen-since to cut shallow repos based on time

This should allow the user to say "create a shallow clone containing the
work from last year" (once the client side is fixed up, of course).

In theory deepen-since and deepen (aka --depth) can be used together to
draw the shallow boundary (whether it's intersection or union is up to
discussion, but if rev-list is used, it's likely intersection). However,
because deepen goes with a custom commit walker, we can't mix the two
yet.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>