I claim the ornamental design for a bar knife, as shown and described.
FIG. 1 is a left side elevation of a handle and a blade for a bar slicing/sawing knife, wherein the right side elevation (not shown) is a mirror image of the left side elevation FIG. 1 of my design;
FIG. 2 is a top plan view of my design;
FIG. 3 is a front elevation view of my design; and,
FIG. 4 is a rear elevation view of my design.
The bottom view is not shown and forms no part of the claimed design.