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 acommodate 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).