git-daemon tests: wait until daemon is ready

git-daemon tests: wait until daemon is ready

In start_daemon, git-daemon is started as a background process.  In
theory, the tests may try to connect before the daemon had a chance
to open a listening socket. Avoid this race condition by waiting
for it to output "Ready to rumble". Any other output is considered
an error and the test is aborted.

Should git-daemon produce no output at all, lib-git-daemon would
block forever. This could be fixed by introducing a timeout.  On
the other hand, we have no timeout for other git commands which
could suffer from the same problem. Since such a mechanism adds
some complexity, I have decided against it.

Signed-off-by: Clemens Buchacher <drizzd@aon.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>