mq: incorporate mq repo config when using --mq
For example, incoming should use the paths defined in .hg/patches/hgrc
instead of .hg/hgrc.
# should print nothing
# should fail
abort: path 'b/x' is inside repo 'b'
abort: path 'b/x' is inside repo 'b'
# should fail
abort: path 'b/x' is inside repo 'b'
# should arguably print nothing
# should fail
abort: path 'b/a' is inside repo 'b'