delta: Support filtering what type of deltas to show

delta: Support filtering what type of deltas to show

Not everyone is interested in every kind of deltas (and some might
even be interested knowing which files do not have overrides), so this
here is an implementation of a --type=LIST... option for
systemd-delta, that makes it possible to filter what subset of deltas
we want.

The available modifiers are masked, equivalent, redirected, overriden,
and unchanged - they should be self explanatory, and the man page
explains them in a little more detail anyway.

As a side effect, in case of overriden files, the diff output was made
optional.

By default, everything is shown (with a diff, if appropriate) except
for completely unchanged files.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>