[Github] Fetch an additional commit for docs CI on PRs

There still seem to be issues with the files changed step taking a
significant amount of time on PRs. This seems to be occurring on PRs
with one commit and not on PRs with more than one commit which is why I
didn't catch this nuance before. Either way, fetching an additional
commit seems to fix the issue. Requires a little bit of hackiness due to
the fact that you can't do math in the github actions variables.

Will be monitoring this over the next little bit to see if this actually
fixes the problem.

CI stuff is annoying sometimes.

Author: boomanaiden154

.github/workflows/docs.yml

@@ -29,14 +29,17 @@ jobs:
     name: "Test documentation build"
     runs-on: ubuntu-latest
     steps:
-      # Fetch all the commits in a pull request so that the
+      # Fetch all the commits in a pull request + 1 so that the
       # docs-changed-subprojects step won't pull them in itself in an extremely
       # slow manner.
-      - name: Fetch LLVM sources (PR)
+      - name: Calculate number of commits to fetch (PR)
         if: ${{ github.event_name == 'pull_request' }} 
+        run: echo "PR_FETCH_DEPTH=$(( ${{ github.event.pull_request.commits }} + 1 ))" >> "${GITHUB_ENV}"
+      - name: Fetch LLVM sources (PR)
+        if: ${{ github.event_name == 'pull_request' }}
         uses: actions/checkout@v4
         with:
-          fetch-depth: ${{ github.event.pull_request.commits }}
+          fetch-depth: ${{ env.PR_FETCH_DEPTH }}
       - name: Fetch LLVM sources (push)
         if: ${{ github.event_name == 'push' }}
         uses: actions/checkout@v4