checkout: error out when index is unmerged even with -m
checkout: error out when index is unmerged even with -m

Even when -m is given to allow fallilng back to 3-way merge
while switching branches, we should refuse if the original index
is unmerged.

Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>
Acked-by: Daniel Barkalow <29008086f408d0a471729b0d8667fa72ead76296@iabervon.org>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git