Change backup policy to be inclusive, meaning all given policies

are evaluated for each snapshot, thereby making sure that each
keep-* is able to retain its most recent snapshot. Thereby insuring
that weeklies keep Sundays around and monthlies keep the last day of
the month around.

Added testcase to make sure when multiple --keep-tags are given,
ALL of them need to match.

doc/manual.rst

@@ -788,6 +788,9 @@ All the ``--keep-*`` options above only count
 hours/days/weeks/months/years which have a snapshot, so those without a
 snapshot are ignored.
 
+All snapshots are evaluated counted against all matching keep-* counts. A
+single snapshot on 30-09-2017 (Sun) will count as a daily, weekly and monthly.
+
 Let's explain this with an example: Suppose you have only made a backup
 on each Sunday for 12 weeks. Then ``forget --keep-daily 4`` will keep
 the last four snapshots for the last four Sundays, but remove the rest.
@@ -796,6 +799,14 @@ is a safety feature: it prevents restic from removing many snapshots
 when no new ones are created. If it was implemented otherwise, running
 ``forget --keep-daily 4`` on a Friday would remove all snapshots!
 
+Another example: Suppose you make daily backups for 100 years. Then
+``forget --keep-daily 7 --keep-weekly 5 --keep-monthly 12 --keep-yearly 75``
+will keep the most recent 7 daily snapshots, then 4 (remember, 7 dailies
+already include a week!) last-day-of-the-weeks and 11 or 12
+last-day-of-the-months. (11 or 12 depends if the 5 weeklies cross a month).
+And ofcourse 75 last-day-of-the-year snapshots. All other snapshots are
+removed.
+
 Autocompletion
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