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>

builtin-diff.c

@@ -223,7 +223,13 @@ static int builtin_diff_files(struct rev_info *revs, int argc, const char **argv
 		argv++; argc--;
 	}
 
-	if (revs->max_count == -1 &&
+	/*
+	 * "diff --base" should not combine merges because it was not
+	 * asked to.  "diff -c" should not densify (if the user wants
+	 * dense one, --cc can be explicitly asked for, or just rely
+	 * on the default).
+	 */
+	if (revs->max_count == -1 && !revs->combine_merges &&
 	    (revs->diffopt.output_format & DIFF_FORMAT_PATCH))
 		revs->combine_merges = revs->dense_combined_merges = 1;