upstream-diff tool: improve help text for command line parameters.

This CL makes the following improvements:
  1. show default values through use of ArgumentDefaultsHelpFormatter;
     it doesn't allow custom newlines, but it didn't seem worth fixing
     with a custom formatter
  2. show possible values for --repositories

Before this CL, the help text for --repositories reads:

  -r REPOSITORIES, --repositories REPOSITORIES
                        Comma-separated list of >= 2 repositories to compare.

After this CL, the help text for --repositories reads:

  -r REPOSITORIES, --repositories REPOSITORIES
                        Comma-separated list of 2-3 repositories, to compare,
                        in order; available repositories: ojluni 7u40
                        8u121-b13 8u60 9b113+ expected. (default:
                        ojluni,expected)

Test: Treehugger

Change-Id: Ide837281aeb342022476d231724db8f48699547b