Revert "v.util: improve color_compare_files (#21243)"

This reverts commit bfd53a9.
It restores the ability to use diff.find_working_diff_command() and
customise the result with VDIFF_TOOL and VDIFF_OPTIONS.

vlib/v/util/diff/diff.v

@@ -65,19 +65,22 @@ fn opendiff_exists() bool {
 	return false
 }
 
-pub fn color_compare_files(diff_cmd string, path1 string, path2 string) string {
-	os.find_abs_path_of_executable(diff_cmd) or {
-		return 'comparison command: `${diff_cmd}` not found'
-	}
-	flags := $if openbsd {
-		['-d', '-a', '-U', '2']
-	} $else $if freebsd {
-		['--minimal', '--text', '--unified=2']
-	} $else {
-		['--minimal', '--text', '--unified=2', '--show-function-line="fn "']
+pub fn color_compare_files(diff_cmd string, file1 string, file2 string) string {
+	if diff_cmd != '' {
+		mut full_cmd := '${diff_cmd} --minimal --text --unified=2  --show-function-line="fn " ${os.quoted_path(file1)} ${os.quoted_path(file2)} '
+		$if freebsd {
+			full_cmd = '${diff_cmd} --minimal --text --unified=2  ${os.quoted_path(file1)} ${os.quoted_path(file2)} '
+		}
+		$if openbsd {
+			full_cmd = '${diff_cmd} -d -a -U 2 ${os.quoted_path(file1)} ${os.quoted_path(file2)} '
+		}
+		x := os.execute(full_cmd)
+		if x.exit_code < 0 {
+			return 'comparison command: `${full_cmd}` not found'
+		}
+		return x.output.trim_right('\r\n')
 	}
-	full_cmd := '${diff_cmd} ${flags.join(' ')} ${os.quoted_path(path1)} ${os.quoted_path(path2)}'
-	return os.execute(full_cmd).output.trim_right('\r\n')
+	return ''
 }
 
 pub fn color_compare_strings(diff_cmd string, unique_prefix string, expected string, found string) string {