unpack_trees: group error messages by type

When an error is encountered, it calls add_rejected_file() which either
- directly displays the error message and stops if in plumbing mode
  (i.e. if show_all_errors is not initialized at 1)
- or stores it so that it will be displayed at the end with display_error_msgs(),

Storing the files by error type permits to have a list of files for
which there is the same error instead of having a serie of almost
identical errors.

As each bind_overlap error combines a file and an old file, a list cannot be
done, therefore, theses errors are not stored but directly displayed.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

tree-walk.h

@@ -45,6 +45,7 @@ struct traverse_info {
 	unsigned long conflicts;
 	traverse_callback_t fn;
 	void *data;
+	int show_all_errors;
 };
 
 int get_tree_entry(const unsigned char *, const char *, unsigned char *, unsigned *);