After the git migration, the synchonization from github to hg.m.o will
add the git commit sha1s as "extra" data in the corresponding Mercurial
changesets.
To support things that use the index to pull artifacts from taskcluster,
like artifact builds off git clones without knowledge about Mercurial,
we need routes based on the git commit sha1s.
This adds such routes, in the same namespace as for the Mercurial
changeset sha1, because it simplifies what the other end needs to do
to accomodate. The chances that a Mercurial changeset sha1 conflict with
a git commit sha1 are pretty slim.
Differential Revision: https://phabricator.services.mozilla.com/D246026
c86fbbc03a