diff: move the fallback "--exit-code" code down

diff: move the fallback "--exit-code" code down

When "--exit-code" is asked and the code cannot just answer by
comparing the object names on both sides but needs to inspect and
compare the contents, there are two ways that the result is found
out.

Some output modes, like "--stat" and "--patch", inherently have to
inspect the contents in order to show the differences in the way
they do.  The codepaths for these modes set the .found_changes bit
as they compute what to show.

However, other output modes do not need to inspect the contents to
show the differences in the way they do.  The most notable example
is "--quiet", which does not need to compute any output to show.
When they are asked to report "--exit-code", they run the codepaths
for the "--patch" output with their output redirected to "/dev/null",
only to set the .found_changes bit.

Currently, this fallback invocation of "--patch" output is done
after the "--stat" output format and its friends and before the
"--patch" and internal callback logic.  Move it to the end of
the sequence to clarify the fallback status of this code block.

Signed-off-by: Junio C Hamano <gitster@pobox.com>