The ornamental design for a shirt, as shown and described.
FIG. 1 is a left-side front perspective view of a shirt showing my new design;
FIG. 2 is a left side view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a right-side front perspective view thereof; and,
FIG. 5 is a rear perspective view thereof.
The broken lines immediately adjacent to the bottom and top ends of 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.