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 <gitster@pobox.com>