builtin-merge: give a proper error message for invalid strategies in config
'git merge -s foobar' diagnosed invalid "foobar" strategy and errored out
with a message, but foobar in pull.twohead or pull.octopus was just
silently ignored. This makes invalid strategy both on the command line
and in the configuration file to trigger the same error.
Signed-off-by: Miklos Vajna <vmiklos@frugalware.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>