Bug 1392886 - Enable sparse extension in Docker images; r=mshal

This needs to be globally enabled to support sparse checkouts
and accessing repos using sparse checkouts.

Having the extension enabled should no-op unless sparse checkouts
are being used. i.e. it should be harmless to globally enable.

MozReview-Commit-ID: AKNUOXfYQPx

--HG--
extra : rebase_source : d32b8a89c56c39923d7b0cd61583c2828a29a872

taskcluster/docker/recipes/install-mercurial.sh

@@ -111,6 +111,7 @@ assume-tty = true
 cacerts = ${CERT_PATH}
 
 [extensions]
+sparse =
 robustcheckout = /usr/local/mercurial/robustcheckout.py
 
 [hostsecurity]