checkout: allow "checkout -m path" to unmerge removed paths

checkout: allow "checkout -m path" to unmerge removed paths

"git checkout -m -- path" uses the unmerge_marked_index() API, whose
implementation is incapable of unresolving a path that was resolved
as removed.  Extend the unmerge_index() API function so that we can
mark the ce_flags member of the cache entries we add to the index as
unmerged, and replace use of unmerge_marked_index() with it.

Now, together with its unmerge_index_entry_at() helper function,
unmerge_marked_index() function is no longer called by anybody, and
can safely be removed.

This makes two known test failures in t2070 and t7201 to succeed.

Signed-off-by: Junio C Hamano <gitster@pobox.com>