Make run-eslint more convenient for local use
The script can be helpful outside of CI, in particular for gradually transitioning to the new style.
Reverting commit f00201fa it is already possible to do something like
$ CI_MERGE_REQUEST_PROJECT_URL=https://gitlab.gnome.org/GNOME/gnome-shell \
CI_MERGE_REQUEST_TARGET_BRANCH_NAME=master CI_COMMIT_SHA=HEAD \
.gitlab-ci/run-eslint.sh
but that is hardly convenient.
Instead, allow passing the required parameters on the command line:
$ .gitlab-ci/run-eslint.sh origin master