rred: Allow passing files as arguments for compressor testing

This introduces a -t mode in which the first argument is input,
the second is output and the remaining are diffs.

This allows us to test patching compressed files, which are
detected using their file extension.

methods/rred.cc

@@ -689,6 +689,7 @@ int main(int argc, char **argv)
 {
    int i;
    bool just_diff = true;
+   bool test = false;
    Patch patch;
 
    if (argc <= 1) {
@@ -696,7 +697,12 @@ int main(int argc, char **argv)
       return Mth.Run();
    }
 
-   if (argc > 1 && strcmp(argv[1], "-f") == 0) {
+   // Usage: rred -t input output diff ...
+   if (argc > 1 && strcmp(argv[1], "-t") == 0) {
+      just_diff = false;
+      test = true;
+      i = 4;
+   } else if (argc > 1 && strcmp(argv[1], "-f") == 0) {
       just_diff = false;
       i = 2;
    } else {
@@ -716,7 +722,13 @@ int main(int argc, char **argv)
       }
    }
 
-   if (just_diff) {
+   if (test) {
+      FileFd out, inp;
+      std::cerr << "Patching " << argv[2] << " into " << argv[3] << "\n";
+      inp.Open(argv[2], FileFd::ReadOnly,FileFd::Extension);
+      out.Open(argv[3], FileFd::WriteOnly | FileFd::Create, FileFd::Extension);
+      patch.apply_against_file(out, inp);
+   } else if (just_diff) {
       FileFd out;
       out.OpenDescriptor(STDOUT_FILENO, FileFd::WriteOnly | FileFd::Create);
       patch.write_diff(out);