The ornamental design for a shirt, as shown and described.
FIG. 1 is a front view of a shirt showing the new design;
FIG. 2 is a right side view thereof;
FIG. 3 is a back side view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a bottom view thereof; and,
FIG. 6 is a side perspective view thereof.
The broken lines shown in the drawings are for the purpose of illustrating portions of the shirt and form no part of the claimed design. A left side view is omitted because that view is a mirror image to the right side view of the new design. The design includes underarm gussets and a yokeless back of a shirt as shown.