checkout: do not check out unmerged higher stages randomly

checkout: do not check out unmerged higher stages randomly

During a conflicted merge when you have unmerged stages for a
path F in the index, if you said:

    $ git checkout F

we rewrote F as many times as we have stages for it, and the
last one (typically "theirs") was left in the work tree, without
resolving the conflict.

This fixes it by noticing that a specified pathspec pattern
matches an unmerged path, and by erroring out.

Signed-off-by: Junio C Hamano <redacted>