t9300-fast-import: store the PID in a variable instead of pidfile
The five tests running 'git fast-import' in the background in
't9300-fast-import.sh' store the PID of that background process in a
pidfile, to be used to check whether that background process survived
each test and then to kill it in test_when_finished commands. To
achieve this all these five tests run three $(cat <pidfile>) command
substitutions each.
Store the PID of the background 'git fast-import' in a variable to
avoid those extra processes.
Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>