What is the disproportionation reaction of ##MgCl##?

There is no disproportionation reaction.

Magnesium has only two : 0 and +2.

Hence, there is no such compound as ##"MgCl"##, in which ##"Mg"## would have an oxidation number of +1.

Even if you meant ##"MgCl"_2##, it still wouldn’t disproportionate.

In a disproportionation reaction, ##"Mg(+2)"## could be reduced to ##"Mg(0)"##, but it would also have to be oxidized to some higher oxidation number, and such a number does not exist.

Therefore, there is no disproportionation reaction.