Doc: clarify wording about tool requirements in sourcerepo.sgml.

Original wording had confusingly vague antecedent for "they", so replace
that with a more repetitive but clearer formulation.  In passing, make the
link to the installation requirements section more specific.  Per gripe
from Martin Mai, though this is not the fix he initially proposed.

Discussion: https://postgr.es/m/CAN_NWRu-cWuNaiXUjV3m4H-riWURuPW=j21bSaLADs6rjjzXgQ@mail.gmail.com