I claim the ornamental design for a bread knife, as shown and described.
FIG. 1 is a left side elevation of a bread knife, wherein the right side elevation (not shown) is a mirror image of the left side elevation of FIG. 1 of my design;
FIG. 2 is a top plan view of my design;
FIG. 3 is an enlarged front elevation view of my design;
FIG. 4 is an enlarged rear elevation view of my design; and,