ci: use dynamic PR base

Previously, we would always compare the diff to master. This is wrong because not all PRs go into master. Instead, we now use the correct SHA of the PR base.

Pull-Request: #4785.

.github/workflows/ci.yml

@@ -85,7 +85,7 @@ jobs:
           ./scripts/ensure-version-bump-and-changelog.sh
         env:
           HEAD_SHA: ${{ github.event.pull_request.head.sha }}
-
+          PR_BASE: ${{ github.event.pull_request.base.ref }}
 
   wasm_tests:
     name: Run all WASM tests

scripts/ensure-version-bump-and-changelog.sh

@@ -5,9 +5,9 @@ set -ex;
 MANIFEST_PATH=$(cargo metadata --format-version=1 --no-deps | jq -e -r '.packages[] | select(.name == "'"$CRATE"'") | .manifest_path')
 DIR_TO_CRATE=$(dirname "$MANIFEST_PATH")
 
-MERGE_BASE=$(git merge-base "$HEAD_SHA" master) # Find the merge base. This ensures we only diff what was actually added in the PR.
+MERGE_BASE=$(git merge-base "$HEAD_SHA" "$PR_BASE") # Find the merge base. This ensures we only diff what was actually added in the PR.
 
-SRC_DIFF_TO_MASTER=$(git diff "$HEAD_SHA".."$MERGE_BASE" --name-status -- "$DIR_TO_CRATE/src" "$DIR_TO_CRATE/Cargo.toml")
+SRC_DIFF_TO_BASE=$(git diff "$HEAD_SHA".."$MERGE_BASE" --name-status -- "$DIR_TO_CRATE/src" "$DIR_TO_CRATE/Cargo.toml")
 CHANGELOG_DIFF=$(git diff "$HEAD_SHA".."$MERGE_BASE" --name-only -- "$DIR_TO_CRATE/CHANGELOG.md")
 
 VERSION_IN_CHANGELOG=$(awk -F' ' '/^## [0-9]+\.[0-9]+\.[0-9]+/{print $2; exit}' "$DIR_TO_CRATE/CHANGELOG.md")
@@ -20,7 +20,7 @@ if [[ "$VERSION_IN_CHANGELOG" != "$VERSION_IN_MANIFEST" ]]; then
 fi
 
 # If the source files of this crate weren't touched in this PR, exit early.
-if [ -z "$SRC_DIFF_TO_MASTER" ]; then
+if [ -z "$SRC_DIFF_TO_BASE" ]; then
   exit 0;
 fi