merge-recursive: handle addition of submodule on our side of history

merge-recursive: handle addition of submodule on our side of history

The code for a newly added path assumed that the path was a normal file,
and thus checked for there being a directory still being in the way of
the file.  Note that since unpack_trees() does path-in-the-way checks
already, the only way for there to be a directory in the way at this
point in the code, is if there is some kind of D/F conflict in the merge.

For a submodule addition on HEAD's side of history, the submodule would
have already been present.  This means that we do expect there to be a
directory present but should not consider it to be "in the way"; instead,
it's the expected submodule.  So, when there's a submodule addition from
HEAD's side, don't bother checking the working copy for a directory in
the way.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>