diff: report unmerged paths as changes in run_diff_cmd()

diff: report unmerged paths as changes in run_diff_cmd()

You can ask the diff machinery to let the exit code indicate whether
there are changes, e.g. with --quiet.  It as two ways to calculate that
bit: The quick one assumes blobs with different hashes have different
content, and the more elaborate way actually compares the contents,
possibly applying transformations like ignoring whitespace.

The quick way considers an unmerged file to be a change and reports
exit code 1, which makes sense.

The slower path uses the struct diff_options member found_changes to
indicate whether the blobs differ even with the transformations applied.
It's not set for unmerged files, though, resulting in exit code 0.

Set found_changes in run_diff_cmd() for unmerged files, for a consistent
exit code of 1 if there's an unmerged file, regardless of whether
whitespace is ignored.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>