fetch/push: readd rsync support

fetch/push: readd rsync support

We lost rsync support when transitioning from shell to C.  Support it
again (even if the transport is technically deprecated, some people just
do not have any chance to use anything else).

Also, add a test to t5510.  Since rsync transport is not configured by
default on most machines, and especially not such that you can write to
rsync://127.0.0.1$(pwd)/, it is disabled by default; you can enable it by
setting the environment variable TEST_RSYNC.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>