fix GotoDiff when no file path is found

Git manual hunk editing files identify as diff, but lack the
file path.
For manual hunk ediiting, it's also useful to use the default
Enter behavior, so we fall back to it if no file path is found.

Fixes #407

diff.py

@@ -141,7 +141,11 @@ def run(self, edit):
         else:
             git_root_dir = ""
 
-        if not os.path.isfile(full_path_file_name):
+        if full_path_file_name is None
+            # If no path exists, this is a manual hunk editing diff
+            # fall back to default "Enter" command
+            self.view.run_command('insert', {'characters':'\n'})
+        elif not os.path.isfile(full_path_file_name):
             caption = "Enter base directory for file '%s':" % self.file_name
             v.window().show_input_panel(caption,
                                         git_root_dir,