Get subparser formatter class from top-level parser

Set a single source of truth.

augur/__init__.py

@@ -52,8 +52,10 @@
 
 def make_parser():
     parser = argparse.ArgumentParser(
-        prog        = "augur",
-        description = "Augur: A bioinformatics toolkit for phylogenetic analysis.")
+        prog            = "augur",
+        description     = "Augur: A bioinformatics toolkit for phylogenetic analysis.",
+        formatter_class = argparse.ArgumentDefaultsHelpFormatter,
+    )
 
     add_default_command(parser)
     add_version_alias(parser)

augur/argparse_.py

@@ -2,7 +2,7 @@
 Custom helpers for the argparse standard library.
 """
 import argparse
-from argparse import Action, ArgumentDefaultsHelpFormatter
+from argparse import Action
 
 
 # Include this in an argument help string to suppress the automatic appending
@@ -58,9 +58,8 @@ def register_commands(parser: argparse.ArgumentParser, commands, command_attribu
         if command_attribute:
             subparser.set_defaults(**{command_attribute: command})
 
-        # Use the same formatting class for every command for consistency.
-        # Set here to avoid repeating it in every command's register_parser().
-        subparser.formatter_class = ArgumentDefaultsHelpFormatter
+        # Ensure all subparsers format like the top-level parser
+        subparser.formatter_class = parser.formatter_class
 
         if not subparser.description and command.__doc__:
             subparser.description = command.__doc__