diffcore-rename: simplify and accelerate register_rename_src()

diffcore-rename: simplify and accelerate register_rename_src()

register_rename_src() took pains to create an array in rename_src which
was sorted by pathname of the contained diff_filepair.  The sorting was
entirely unnecessary since callers pass filepairs to us in sorted
order.  We can simply append to the end of the rename_src array,
speeding up diffcore_rename() setup time.

Also, note that I dropped the return type on the function since it was
unconditionally discarded anyway.

This patch is being submitted in a different order than its original
development, but in a large rebase of many commits with lots of renames
and with several optimizations to inexact rename detection,
diffcore_rename() setup time was a sizeable chunk of overall runtime.
This patch dropped execution time of rebasing 35 commits with lots of
renames by 2% overall.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>