http-push.c: remove duplicated code

http-push.c: remove duplicated code

An earlier commit aa1dbc9 (Update http-push functionality, 2006-03-07)
borrowed some code from rev-list.c.

This copy and paste made sense back then, because mark_edges_uninteresting(),
and its helper mark_edge_parents_uninteresting(), accessed a file scope
static variable  "revs" in rev-list.c, and http-push.c did not have nor care
about such a variable.

But these days they are already properly libified and live in list-objects.c
and they take "revs" as as an argument.  Make use of them and lose 20 or
so lines.

Signed-off-by: Junio C Hamano <gitster@pobox.com>