unpack-trees: check if we can perform the operation for submodules

unpack-trees: check if we can perform the operation for submodules

In a later patch we'll support submodule entries to be
in sync with the tree in working tree changing commands,
such as checkout or read-tree.

When a new submodule entry changes in the tree, we need to
check if there are conflicts (directory/file conflicts)
for the tree. Add this check for submodules to be
performed before the working tree is touched.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>