Bug 1230610 - Switch Hg pushlog ingestion to using fetch_json()
Bug 1230610 - Switch Hg pushlog ingestion to using fetch_json()

To save reinventing the wheel, particularly once we start adding a
custom user agent to the request headers.

Later it might be worth moving logging into `fetch_json()` too, however
for now the various consumers of `fetch_json()` have different
interpretations of whether an `HTTPError` is really an error, which
means there isn't one size fits all, so let's leave that to another bug.

kapy2010/treeherder,tojonmz/treeherder,glenn124f/treeherder,tojonmz/treeherder,tojonmz/treeherder,akhileshpillai/treeherder,akhileshpillai/treeherder,tojon/treeherder,edmorley/treeherder,wlach/treeherder,glenn124f/treeherder,avih/treeherder,akhileshpillai/treeherder,kapy2010/treeherder,akhileshpillai/treeherder,KWierso/treeherder,avih/treeherder,akhileshpillai/treeherder,avih/treeherder,wlach/treeherder,edmorley/treeherder,akhileshpillai/treeherder,glenn124f/treeherder,KWierso/treeherder,tojon/treeherder,tojonmz/treeherder,edmorley/treeherder,kapy2010/treeherder,wlach/treeherder,wlach/treeherder,glenn124f/treeherder,tojon/treeherder,tojonmz/treeherder,edmorley/treeherder,KWierso/treeherder,kapy2010/treeherder,avih/treeherder,wlach/treeherder,glenn124f/treeherder,tojon/treeherder,avih/treeherder,wlach/treeherder,tojonmz/treeherder,avih/treeherder,kapy2010/treeherder,KWierso/treeherder,glenn124f/treeherder