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
the trick.

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