t/lib-git-daemon: make sure to kill the 'git-daemon' process

After 'start_git_daemon' starts 'git daemon' (note the space in the middle) in the background, it saves the background process' PID, so the daemon can be stopped at the end of the test script. However, 'git-daemon' is not a builtin but a dashed external command, which means that the dashless 'git daemon' executes the dashed 'git-daemon' command, and, consequently, the PID recorded is not the PID of the "real" daemon process, but that of the main 'git' wrapper. Now, if a test script involving 'git daemon' is interrupted by ctrl-C, then only the main 'git' process is stopped, but the real daemon process tends to survive somehow, and keeps on running in the background indefinitely, keeping the daemon's port to itself, and thus preventing subsequent runs of the same test script. Work this around by running 'git daemon' with the '--pidfile=...' option to save the PID of the real daemon process, and kill that process in 'stop_git_daemon' as well. Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>