The ornamental design for a christmastreehalo, as shown and described.
FIG. 1 is a top perspective view of a first embodiment of a Christmas tree halo showing my new design;
FIG. 2 is a bottom perspective view thereof;
FIG. 3 is a top perspective view of a second embodiment of a Christmas tree halo in an alternate bent configuration showing my new design;
FIG. 4 is a front view thereof;
FIG. 5 is a back view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof;
FIG. 8 is a left view thereof; and,
FIG. 9 is a right view thereof.
The broken lines showing of connectors in FIG. 1-9 are included for the purpose of showing environmental structure and form no part of the claimed design.