I claim the ornamental design for a chisel, as shown and described.
FIG. 1 is a perspective view of a chisel, showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top plan view thereof; and
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is another perspective view of the chisel wherein the broken line showing of the bit is for environmental purposes only and forms no part of the claimed design.