Merge branch 'nd/reject-empty-shallow-request'

"git fetch --shallow-since=<cutoff>" that specifies the cut-off
point that is newer than the existing history used to end up
grabbing the entire history.  Such a request now errors out.

* nd/reject-empty-shallow-request:
  upload-pack: reject shallow requests that would return nothing

diff --cc shallow.c
Simple merge

diff --cc t/t5500-fetch-pack.sh
Simple merge