The ornamental design for a shirt, as shown and described.
FIG. 1 is a front right perspective view of a shirt showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The grayscale photographic area forms part of the claimed design. The broken lines immediately adjacent to the grayscale photographic area represent unclaimed boundaries of the design. The broken lines showing the remainder of the garment depict environment only and form no part of the claimed design.