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 left side view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a rear view thereof; and,
FIG. 6 is a rear perspective view thereof.
The contrast in shading represents a contrast of appearance and no specific colors, textures, or patterns are hereby claimed. The broken lines within the shaded areas form no part of the claimed design. The broken lines depicting the remainder of the shirt show features that form no part of the claimed design.