t1100: move creation of expected output into setup test

t1100: move creation of expected output into setup test

The "expected" file is created at the top-level of the script, outside
of any test. Code that runs outside of a test is not protected by the
test harness: a failure there is not reported as a test failure and is
easy to miss.

Move the here-doc that creates "expected" into the existing setup test
("test preparation: write empty tree"), using a "<<-" here-doc so its
body can be indented along with the rest of the test.

Signed-off-by: Shlok Kulshreshtha <redacted>
Signed-off-by: Junio C Hamano <redacted>