uncrustify: use unified diff to generate GH annotation

`git diff` creates diffs where the file name starts with `a/` or `b/`
respectively. These are obviously not in path, so the Github annotations
point to nowhere when the path is used without accounting for this.

This fixes the GH annotation parsing to account for that.

dist/tools/uncrustify/uncrustify.sh

@@ -57,7 +57,8 @@ exec_uncrustify () {
                 if echo "$line" | grep -q '^--- .\+$'; then
                     _annotate_diff "$DIFFFILE" "$DIFFLINE" "$DIFF"
                     DIFF="$line"
-                    DIFFFILE=$(echo "$line" | sed 's/^--- \(.\+\)$/\1/g')
+                    DIFFFILE=$(echo "$line" |
+                        sed 's#^--- \([ab]/\)\?\(.\+\)$#\2#g')
                     DIFFLINE=""
                 # we are in a diff currently
                 elif [ -n "$DIFF" ]; then