Revision 1917632

Clarify/document some parameter processing, particular around groups
that specify they only apply to particular repositories (via the
"for_repos" config value).

Lots of comments added to clarify the processing/handling.

* tools/hook-scripts/mailer/mailer.py:
  (main): pass REPOS_DIR rather than REPOS to the Config constructor.
    It only needs the directory, not the repository object.
  (Config.__init__): accept the REPOS_DIR instead of a Repository
    object. Rename GLOBAL_PARAMS to DEFAULT_PARAMS for consistency in
    some later naming. Omit saving the (old) GLOBAL_PARAMS concept, as
    nobody ever uses that. Pass the DEFAULT_PARAMS into prep_groups(),
    and update to pass REPOS_DIR.
  (Config._prep_groups): revise params to take REPOS_DIR and
    DEFAULT_PARAMS (instead of using the old dropped-off globals).
    Add new repos_params() function to look for a "for_repos" config
    value to see if this group/section applies to the repository
    specified by REPOS_DIR. Replaces two similar hunks of code in this
    method. Use repos_params() for the self._default_params and each
    group/section found in the configuration file.