Refactor --dirstat parsing; deprecate --cumulative and --dirstat-by-file

Refactor --dirstat parsing; deprecate --cumulative and --dirstat-by-file

Instead of having multiple interconnected dirstat-related options, teach
the --dirstat option itself to accept all behavior modifiers as parameters.

 - Preserve the current --dirstat=<limit> (where <limit> is an integer
   specifying a cut-off percentage)
 - Add --dirstat=cumulative, replacing --cumulative
 - Add --dirstat=files, replacing --dirstat-by-file
 - Also add --dirstat=changes and --dirstat=noncumulative for specifying the
   current default behavior. These allow the user to reset other --dirstat
   parameters (e.g. 'cumulative' and 'files') occuring earlier on the
   command line.

The deprecated options (--cumulative and --dirstat-by-file) are still
functional, although they have been removed from the documentation.

Allow multiple parameters to be separated by commas, e.g.:
  --dirstat=files,10,cumulative

Update the documentation accordingly, and add testcases verifying the
behavior of the new syntax.

Improved-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>