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>

unpack-trees.h

@@ -21,6 +21,7 @@ enum unpack_trees_error_types {
 	ERROR_SPARSE_NOT_UPTODATE_FILE,
 	ERROR_WOULD_LOSE_ORPHANED_OVERWRITTEN,
 	ERROR_WOULD_LOSE_ORPHANED_REMOVED,
+	ERROR_WOULD_LOSE_SUBMODULE,
 	NB_UNPACK_TREES_ERROR_TYPES
 };