Only run linkcheck for changed source files in PRs

[JoergAtGithub]

This still runs the full link-check over all source files on push commits, but in PRs it only checks th links in files source files changed compared to the base commit of the PR.
Means if we want to see the full list of broken links, we can always look at the resul of branches like 2.5, 2.6 or main, while the CI result of PR buils will be green unless a link in the modified fils is broken.

[JoergAtGithub]

I tested this with injected failures in this PR. Finally it worked as intended and I reverted the injected failures.

[acolombier]

Thanks for addressing this. I have suggested a bash-only approach, that could allow removing the Python script and allow to easily run it locally when debugging

[acolombier]

Suggested change

	- name: Fetch base branch for diff
	if: github.event_name == 'pull_request'
	run: git fetch origin ${{ github.event.pull_request.base.sha }}
	- name: Get changed source files
	id: changed
	if: github.event_name == 'pull_request'
	env:
	GITHUB_EVENT_NAME: ${{ github.event_name }}
	PR_BASE_SHA: ${{ github.event.pull_request.base.sha }}
	PUSH_BEFORE_SHA: ${{ github.event.before }}
	run: python tools/list_changed_source_files.py
	- name: Install Python dependencies
	if: github.event_name == 'push' || steps.changed.outputs.files != ''
	run: |
	python -m pip install --upgrade pip
	python -m pip install -r requirements.txt
	- name: Check for broken links
	run: sphinx-build -q --color -b linkcheck source build
	if: github.event_name == 'push' || steps.changed.outputs.files != ''
	run: sphinx-build -q --color -b linkcheck source build ${{ steps.changed.outputs.files }}
	- name: Fetch base branch for diff
	if: github.event_name == 'pull_request'
	run: git fetch origin ${{ github.event.pull_request.base.sha }}
	- name: Install Python dependencies
	if: github.event_name == 'push' || steps.changed.outputs.files != ''
	run: |
	python -m pip install --upgrade pip
	python -m pip install -r requirements.txt
	- name: Check for broken links
	env:
	BEFORE: ${{ github.event_name == 'pull_request' && github.event.pull_request.base.sha || github.event.before }}
	run: |
	changed_files=`git diff $BEFORE --name-only --diff-filter=d HEAD -- source/ | grep -E '\.(md|rst)$' | xargs`
	if [ -n "$changed_files" ]; then
	sphinx-build -q --color -b linkcheck source build $changed_files
	fi

[JoergAtGithub]

bash is Unix only. Not a good option for a platform Independent project.

[acolombier]

Does git not come with Bash anymore on Windows?