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

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>