xdiff-interface: prepare for allowing early return

xdiff-interface: prepare for allowing early return

Change the function prototype of xdiff_emit_line_fn to return an "int"
instead of "void". Change all of those functions to "return 0",
nothing checks those return values yet, and no behavior is being
changed.

In subsequent commits the interface will be changed to allow early
return via this new return value.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>