t9300-fast-import: don't hang if background fast-import exits too early

The five tests checking 'git fast-import's checkpoint handling in 't9300-fast-import.sh', all with the prefix "V:" in their test description, can hang indefinitely if 'git fast-import' unexpectedly dies early in any of these tests. These five tests run 'git fast-import' in the background, while feeding instructions to its standard input through a fifo (fd 8) from a background subshell, and reading and verifying its standard output through another fifo (fd 9) in the test script's main shell process. This "reading and verifying" is basically a 'while read ...' shell loop iterating until 'git fast-import' outputs the expected line, ignoring any other output. This doesn't work very well when 'git fast-import' dies before printing that particular line, because the 'read' builtin doesn't get EOF after the death of 'git fast-import', as their input and output are not connected directly but through a fifo. Consequently, that 'read' hangs waiting for the next line from the already dead 'git fast-import', leaving the test script and in turn the whole test suite hanging. Avoid this hang by checking whether the background 'git fast-import' process exited unexpectedly early, and interrupt the 'while read' loop if it did. We have to jump through some hoops to achive that, though: - Start the background 'git fast-import' in another background subshell, which then: - prints the PID of that 'git fast-import' process to the fifo, to be read by the main shell process, so it will know which process to kill when the test is finished. - waits until that 'git fast-import' process exits. If it does exit, then report its exit code, and write a message to the fifo used for 'git fast-import's standard output, thus un-block the 'read' builtin in the main shell process. - Modify that 'while read' loop to break the loop upon seeing that message, and fail the test in the usual way. - Once the test is finished kill that background subshell as well, and do so before killing the background 'git fast-import'. Otherwise the background 'git fast-import' and subshell processes would die racily, and if 'git fast-import' were to die sooner, then we might get some undesired and potentially confusing messages in the test's output. Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>