diff: correctly disable external_diff with --no-ext-diff

diff: correctly disable external_diff with --no-ext-diff

Upon seeing a type-change filepair, "diff --no-ext-diff" does not
show the usual "deletion followed by addition" split patch and does
not run the external diff driver either.

This is because the logic to disable external diff was placed at a
wrong level in the callchain.  run_diff_cmd() decides to show the
split patch only when external diff driver is not configured or
specified via GIT_EXTERNAL_DIFF environment, but this is done before
checking if --no-ext-diff was given.  To make things worse,
run_diff_cmd() checks --no-ext-diff and disables the output for such
a filepair completely, as the callchain below it (e.g. builtin_diff)
does not want to handle typechange filepairs.

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