Plan PR builds from the PR's own files, not a diff to master's tip

The planner took 'git diff base.sha head.sha', a two-dot diff between
the current master tip and the PR head. For a PR that branched before
later merges, that counts every package master has changed since, so
the PR plans those too and builds its own stale copies of them: wasted
builds, and 'already up to date' Pack failures that turn the PR red for
packages it never touched. #397 (lazyjournal) planned 22 packages for a
one-package change. The checkout is shallow, so ask GitHub for the PR's
files, which are listed against the merge base.

.github/workflows/build-pr.yml

@@ -95,8 +95,13 @@ jobs:
           if [[ -n "${{ github.event.inputs.packages }}" ]]; then
             names="${{ github.event.inputs.packages }}"
           else
+            # The PR's own files, as GitHub lists them against the merge base.
+            # A two-dot diff against the current base tip also counted every
+            # package master changed after the PR branched, so a stale PR
+            # planned dozens of unrelated packages at its old versions. The
+            # checkout here is shallow, so there is no merge base to diff from.
             # A package the PR deletes has nothing to build.
-            names=$(git diff --name-only "${{ github.event.pull_request.base.sha }}" "${{ github.event.pull_request.head.sha }}" -- pkgbuilds \
+            names=$(gh api --paginate "repos/${{ github.repository }}/pulls/${{ github.event.pull_request.number }}/files" --jq '.[].filename' \
               | awk -F/ '$1=="pkgbuilds" && NF>2 {print $2}' | sort -u \
               | while read -r name; do
                   if git cat-file -e "${{ github.event.pull_request.head.sha }}:pkgbuilds/$name" 2>/dev/null; then echo "$name"; fi