diff/diff-files: do not use --cc too aggressively

diff/diff-files: do not use --cc too aggressively

Textual diff output for unmerged paths was too eager to give condensed
combined diff.  Even though "diff -c" (and "diff-files -c -p") is a
request to view combined diff without condensing (otherwise the user would
have explicitly asked for --cc, not -c), we showed "--cc" output anyway.

0fe7c1d (built-in diff: assorted updates, 2006-04-29) claimed to be
careful about doing this, but its breakage was hidden because back then
"git diff" was still a shell script that did not use the codepath it
introduced fully.

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