* `Some(x).unwrap()` is not necessary.
* Add a new test to ensure that we handle the fix child process IO
without deadlock. The new test introduces byte mode to fake-formatter
which pipe the stdin contents to stdout byte by byte in a streaming
fashion. The test contents are 512KB, which is larger than page size,
can trigger the blocking read behavior, and doesn't make the test the
slowest.
This allows tests to easily distinguish the effects of multiple formatters that
could be applied during a single `jj fix` command, which is helpful for testing
the feature for configuring multiple formatters on potentially overlapping
filesets.
Uppercase is a counterpart to lowercase. Append exposes the number of changes
and their order of execution in the file content, which provides a terse way of
writing test expectations.
