Test "stg status" with -M in stgit.diff-opts

It used to fail, before the recent merge of the -O/--diff-opts
removal, since the default value of that option was taken from
stgit.diff-opts and passed on to stgit.git.tree_status() which
couldn't handle it.

Signed-off-by: Karl Hasselström <kha@treskal.com>