Avoid generating review drafts for merge commits

This avoids generating review drafts for merge commits unless the merge commit has modifications to the review draft (which probably means that it's different from *all* parents of the merge).

The code prior to this change tries to generate a review draft if the first parent of the merge commit introduces a review draft.  (This happens if "git merge main" is done on a feature branch and main has introduced a review draft.)

Author: dbaron

build.sh

@@ -87,7 +87,7 @@ function main {
   if [[ -e "$HTML_GIT_DIR" ]]; then
     # This is based on https://github.com/whatwg/whatwg.org/pull/201 and should be kept synchronized
     # with that.
-    CHANGED_FILES=$(git --git-dir="$HTML_GIT_DIR" diff --name-only HEAD^ HEAD)
+    CHANGED_FILES=$(git --git-dir="$HTML_GIT_DIR" show --format="format:" --name-only HEAD)
     for CHANGED in $CHANGED_FILES; do # Omit quotes around variable to split on whitespace
       if ! [[ "$CHANGED" =~ ^review-drafts/.*.wattsi$ ]]; then
         continue