Fix interdiff context trimming when output is colorized

When interdiff's output is set as colorized, that means the output from the
diff execution inside interdiff contains color escape codes. The color
escape codes break trim_context(), which doesn't expect a line to start
with an escape character.

Fix trim_context() to handle colorized output from diff.

Author: kerneltoast

src/interdiff.c

@@ -988,18 +988,28 @@ trim_context (FILE *f /* positioned at start of @@ line */,
 
 		if (read_atatline (line, &orig_offset, &orig_count,
 				   &new_offset, &new_count))
-			error (EXIT_FAILURE, 0, "Line not understood: %s",
-			       line);
+			goto line_not_understood;
 
 		orig_orig_count = new_orig_count = orig_count;
 		orig_new_count = new_new_count = new_count;
 		fgetpos (f, &pos);
 		while (orig_count || new_count) {
+			char *first_char;
+
 			if (getline (&line, &linelen, f) < 0)
 				break;
 
+			/* Skip past any color escape code for a +/- line */
+			if (line[0] == 0x1b) {
+				first_char = strpbrk(line, "+-");
+				if (!first_char)
+					goto line_not_understood;
+			} else {
+				first_char = line;
+			}
+
 			total_count++;
-			switch (line[0]) {
+			switch (*first_char) {
                         case '\n':
                                 whitespace_damage("input");
 			case ' ' :
@@ -1081,6 +1091,10 @@ trim_context (FILE *f /* positioned at start of @@ line */,
 	error (0, 0, "hunk-splitting is required in this case, but is not yet implemented");
 	error (1, 0, "use the -U option to work around this");
 	return 0;
+
+ line_not_understood:
+	error (EXIT_FAILURE, 0, "Line not understood: %s", line);
+	return 0;
 }
 
 static int