let git grep consider sub projects

How can I teach 'git grep $string' to also consider subprojects?

The build system of xen.git clones 4 other trees into a directory in its
source tree during build. It would be nice if my 'git grep' searches
also in these cloned subdirs. Somehow there must be some knowledge
because 'git clean -dfx' leaves them alone, while 'git clean -dffx'
wipes everything.

Olaf

PS: Sometimes I miss a 'git clean -dfx --also-sub-repos' useful to
really clean everything before starting over.