Add a scope list to each team. This list defines all the files and
directories that are mentored by the team.
Also add a cc-members command that takes two Git revision strings as input,
add returns the members that should be CC'ed given the files impacted between
the two revisions.
* etc/teams.scm.in (<team>)[scope]: New field.
(team, list-teams): Adapt those procedures.
(find-team-by-scope, diff-revisions): New procedures.
(main): Add a "cc-members" command.
* doc/contributing.texi ("Teams"): Document it.
("Sending a Patch Series"): Adapt it.
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