The ornamental design for a shirt, as shown and described.
FIG. 1 is a front perspective view of a shirt showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a rear view thereof; and,
FIG. 5 is a left side view thereof.
The broken lines immediately adjacent to the shaded areas define the bounds of the claimed design and form no part thereof. The broken lines depicting the remainder of the shirt show features that form no part of the claimed design.