The error is easy to miss, because the test passes and stderr is not
printed. But if you run basic tests in verbose mode (./basic
--verbose), you get:
sed: can't read notmuch-test: No such file or directory
The issue is that sed command is given two files: notmuch-test and
$TEST_DIRECTORY/notmuch-test. And there is no notmuch-test file in
the current directory (test/tmp.basic/). The patch just removes the
non-existing file from the sed command.