Clarify when an external command emits for both files, otherwise it can look...

Clarify when an external command emits for both files, otherwise it can look like diffoscope is repeating itself when it is actually being run twice.

diffoscope/difference.py

@@ -297,12 +297,12 @@ class Difference:
 
         if command1 and command1.stderr:
             difference.add_comment(
-                "stderr from `{}`:".format(command1.shell_cmdline())
+                "stderr from `{}` (a):".format(command1.shell_cmdline())
             )
             difference.add_comment(command1.stderr)
         if command2 and command2.stderr:
             difference.add_comment(
-                "stderr from `{}`:".format(command2.shell_cmdline())
+                "stderr from `{}` (b):".format(command2.shell_cmdline())
             )
             difference.add_comment(command2.stderr)