Dangerfile: Fixed how changed files are determined (#2757)

dangerfile.js

@@ -24,6 +24,25 @@ function readPRFile(path) {
 	return git.show([`pr:${path}`]);
 }
 
+/**
+ * Returns the relative paths of all files changed in the PR.
+ *
+ * @returns {Promise<string[]>}
+ */
+const getChangedFiles = async () => {
+	// Determine the merge base between master and the PR branch.
+	// If files changed in master since PR was branched they would show in the diff otherwise.
+	// https://stackoverflow.com/questions/25071579/list-all-files-changed-in-a-pull-request-in-git-github
+	const mergeBase = await git.raw(['merge-base', 'pr', 'HEAD']);
+	const result = await git.diff(['--name-only', '--no-renames', 'pr', mergeBase]);
+	return (result || '').split(/\r?\n/g);
+};
+
+const getChangedMinifiedFiles = async () => {
+	const changed = await getChangedFiles();
+	return changed.filter(file => file.endsWith('.min.js'));
+};
+
 // https://stackoverflow.com/questions/15900485/correct-way-to-convert-size-in-bytes-to-kb-mb-gb-in-javascript
 const formatBytes = (bytes, decimals = 2) => {
 	if (bytes === 0) return '0 Bytes';
@@ -64,11 +83,6 @@ const getSummary = (rows, totalMasterFileSize, totalFileSize) => {
 	return `A total of ${numFiles} file${maybeS} have changed, with a combined diff of ${byteDiff} (${percentDiff}).`;
 }
 
-const getChangedMinifiedFiles = async () => {
-	const result = await git.diff(['--name-only', '--no-renames', 'pr', 'HEAD']);
-	return (result || '').split(/\r?\n/g).filter(file => file.endsWith('.min.js'));
-};
-
 const run = async () => {
 	const minified = await getChangedMinifiedFiles();