t6026: clarify the point of "kill $(cat sleep.pid)"

We lengthened the time the leftover process sleeps in the previous
commit to make sure it will be there while 'git merge' runs and
finishes.  It therefore needs to be killed before leaving the test.
And it needs to be killed even when 'git merge' fails, so it has to
be triggered via test_when_finished mechanism.

Explain all that in a large comment, and move the use site of
test_when_finished to immediately before 'git merge' invocation,
where the process is spawned.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6026-merge-attr.sh

@@ -187,12 +187,20 @@ test_expect_success 'custom merge does not lock index' '
 		sleep 3600 &
 		echo $! >sleep.pid
 	EOF
-	test_when_finished "kill \$(cat sleep.pid)" &&
 
 	test_write_lines >.gitattributes \
 		"* merge=ours" "text merge=sleep-an-hour" &&
 	test_config merge.ours.driver true &&
 	test_config merge.sleep-an-hour.driver ./sleep-an-hour.sh &&
+
+	# We are testing that the custom merge driver does not block
+	# index.lock on Windows due to an inherited file handle.
+	# To ensure that the backgrounded process ran sufficiently
+	# long (and has been started in the first place), we do not
+	# ignore the result of the kill command.
+	# By packaging the command in test_when_finished, we get both
+	# the correctness check and the clean-up.
+	test_when_finished "kill \$(cat sleep.pid)" &&
 	git merge master
 '