grep: allocate subrepos on heap

grep: allocate subrepos on heap

Currently, struct repository objects corresponding to submodules are
allocated on the stack in grep_submodule(). This currently works because
they will not be used once grep_submodule() exits, but a subsequent
patch will require these structs to be accessible for longer (perhaps
even in another thread). Allocate them on the heap and clear them only
at the very end.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Reviewed-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>