remote: improve sorting of "configure for git push" list

remote: improve sorting of "configure for git push" list

The data structure used to store this list is a string_list
of sources with the destination in the util member. The
current code just sorts on the source; if a single source is
pushed to two different destination refs at a remote, then
the order in which they are printed is non-deterministic.

This patch implements a comparison using both fields.
Besides being a little nicer on the eyes, giving a stable
sort prevents false negatives in the test suite when
comparing output.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>