builtin/maintenance: don't silently ignore invalid strategy

builtin/maintenance: don't silently ignore invalid strategy

When parsing maintenance strategies we completely ignore the
user-configured value in case it is unknown to us. This makes it
basically undiscoverable to the user that scheduled maintenance is
devolving into a no-op.

Change this to instead die when seeing an unknown maintenance strategy.
While at it, pull out the parsing logic into a separate function so that
we can reuse it in a subsequent commit.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Acked-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>