fix_doc_only_skip

[hl475]

Summary

My PR #180 testing is wrong - i used a commit which is already in main to test (which uses get_diff_main instead of get_diff)

Test

Test on PR with changes on both docs and others

vllm-project/vllm#25741
=>
https://buildkite.com/vllm/ci/builds/33142/steps/canvas

Test on PR with changes on docs only

vllm-project/vllm#26024
=>
https://buildkite.com/vllm/ci/builds/33144/steps/canvas

Test on PR with no changes on docs

vllm-project/vllm#26053
=>
https://buildkite.com/vllm/ci/builds/33149/steps/canvas

Test on merge PR with changes on both docs and others

vllm-project/vllm@2f652e6
=>
https://buildkite.com/vllm/ci/builds/33148/steps/canvas

Test on merged PR with changes on docs only

vllm-project/vllm@57b46d7
=>
https://buildkite.com/vllm/ci/builds/33147/steps/canvas

Test on merged PR with no changes on docs

vllm-project/vllm@c36f0aa
=>
https://buildkite.com/vllm/ci/builds/33150/steps/canvas