Add old path to NumStats protobuf output

This will make it possible to solve some N+1 issues in the /internal/allowed endpoint.

Relates to gitlab#227572 (closed)

Test data was updated via:

output = File.open('internal/diff/testdata/z-numstat.txt', "ab")
data = "0\t0\t\0files/original.jpg\0files/new.jpg\0"
output.write(data)
output.close