The ornamental design for the baseball cap shirt, as shown and described.
FIG. 1 is a front view of a baseball cap shirt showing my new design;
FIG. 2 is a left side view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a front perspective view thereof;
FIG. 6 is a back perspective view thereof;
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view thereof.
The short dash lines on the shirt are claimed stitching. The long broken lines represent unclaimed subject matter and represent possible short sleeve boundaries. The dash-dot-dash lines represent the long sleeve bounds of the claimed design and represent no part of the claimed design.