Fix neglect of diff_setup()/diff_setup_done() symmetry.

Fix neglect of diff_setup()/diff_setup_done() symmetry.

Code that calls diff_setup(), including via init_revisions(), should later call
diff_setup_done(), possibly via setup_revisions(). Failure to do so could cause
errors, especially in the future when we add responsibilities to
diff_setup_done(). This instance causes no known errors with the present code.
But it resulted in an error with an experimental patch.

Signed-off-by: Keith Cascio <redacted>
Signed-off-by: Junio C Hamano <redacted>