gitlab-ci: Disable shallow clones
Previously we were passing --unshallow to git fetch in the linting
rules to ensure that the base commit which we were linting with respect
to was available. However, this breaks due to GitLab's re-use of
working directories since git fetch --unshallow fails on a repository
which is not currently shallow.
Given that git fetch --unshallow circumvents the efficiencies provided
by shallow clones anyways, let's just disable them entirely.
There is no documented way to do disable shallow clones but on checking
the GitLab implementation it seems that setting GIT_DEPTH=0 should do