Pre-fetch neighboring files when single file-by-file file is displayed

app/assets/javascripts/diffs/components/diff_file.vue

app/assets/javascripts/diffs/store/actions.js

app/assets/javascripts/diffs/store/mutation_types.js

app/assets/javascripts/diffs/store/mutations.js

app/assets/javascripts/diffs/store/utils.js

app/assets/javascripts/diffs/utils/tree_worker_utils.js

spec/frontend/diffs/components/diff_file_spec.js

spec/frontend/diffs/store/actions_spec.js

spec/frontend/diffs/store/mutations_spec.js

spec/frontend/diffs/store/utils_spec.js

spec/frontend/diffs/utils/tree_worker_utils_spec.js