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>

builtin/checkout.c

@@ -392,6 +392,7 @@ static int merge_working_tree(struct checkout_opts *opts,
 		topts.dir = xcalloc(1, sizeof(*topts.dir));
 		topts.dir->flags |= DIR_SHOW_IGNORED;
 		topts.dir->exclude_per_dir = ".gitignore";
+		topts.show_all_errors = 1;
 		tree = parse_tree_indirect(old->commit ?
 					   old->commit->object.sha1 :
 					   (unsigned char *)EMPTY_TREE_SHA1_BIN);