I claim the ornamental design for a shirt, as shown and described.
FIG. 1 is a front elevational view of a shirt, showing my new design;
FIG. 2 is an enlarged front elevational view thereof;
FIG. 3 is a rear elevational view of FIG. 2;
FIG. 4 is a right-side elevational view of FIG. 2, the left-side elevational view being a mirror image thereof;
FIG. 5 is a top plan view of FIG. 2; and,
FIG. 6 is a bottom plan view of FIG. 2.
The broken lines showing a person in FIG. 1 and portions of the shirt in the figures depict environmental matter and form no part of the claimed design. The broken dot-dash line represents a boundary of the claimed design. The broken lines within the boundary of the claimed area represent stitching and form a part of the claimed design.