Feature request: 'git grep' over multiple working trees

If the current directory contains several git working trees (each in its own subdirectory) then 'git grep' should search all of them.  It would be roughly equivalent to

% for i in * ; do (cd $i ; git grep whatever); done

except that the filenames listed would have the full path.

As you see it's fairly simple to script up, and there are tools to do it (one called 'multi'), but having it built in to 'git grep' would be handy. If the current directory doesn't contain any git working trees, then 'git grep' can die as it does now. If there are some working trees and some random other stuff, it can warn and skip the other stuff.

Please ignore confidentiality stuff after this point.