t/lib-git-daemon: record daemon log

When we start git-daemon for our tests, we send its stderr
log stream to a named pipe. We synchronously read the first
line to make sure that the daemon started, and then dump the
rest to descriptor 4. This is handy for debugging test
output with "--verbose", but the tests themselves can't
access the log data.

Let's dump the log into a file, as well, so that future
tests can check the log. There are a few subtleties worth
calling out here:

  - we'll continue to send output to descriptor 4 for
    viewing/debugging, which would imply swapping out "cat"
    for "tee". But we want to ensure that there's no
    buffering, and "tee" doesn't have a standard way to
    ask for that. So we'll use a shell loop around "read"
    and "printf" instead. That ensures that after a request
    has been served, the matching log entries will have made
    it to the file.

  - the existing first-line shell loop used read/echo. We'll
    switch to consistently using "read -r" and "printf" to
    relay data as faithfully as possible.

  - we open the logfile for append, rather than just output.
    That makes it OK for tests to truncate the logfile
    without restarting the daemon (the OS will atomically
    seek to the end of the file when outputting each line).
    That allows tests to look at the log without worrying
    about pollution from earlier tests.

Helped-by: Lucas Werkmeister <mail@lucaswerkmeister.de>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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