clone: send diagnostic messages to stderr

Putting messages like "Cloning into.." and "done" on stdout is un-Unix and uselessly clutters the stdout channel. Send them to stderr. We have to tweak two tests to accommodate this: 1. t5601 checks for doubled output due to forking, and doesn't actually care where the output goes; adjust it to check stderr. 2. t5702 is trying to test whether progress output was sent to stderr, but naively does so by checking whether stderr produced any output. Instead, have it look for "%", a token found in progress output but not elsewhere (and which lets us avoid hard-coding the progress text in the test). This should not regress any scripts that try to parse the current output, as the output is already internationalized and therefore unstable. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>