merge-tree: fix "same file added in subdir"

When the same file is added with identical content at the top level,
git-merge-tree prints "added in both" with the details.  But if the file
is added in an existing subdirectory, threeway_callback() happily returns
silently as if there is no conflict (as both sides added the same).

In order to detect this, we need to fall through and recurse into
the subtree in this case.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'jk/merge-tree-added-identically' into pu

* jk/merge-tree-added-identically:
  merge-tree: fix "same file added in subdir"