check: Use --text when potentially diffing generated files

The diffs generated from running `tools/check build_runner` were not
detected.  See comment:
zulip#904 (comment)

We normally exclude diffs from generated files through our
`.gitattributes` configurations.  This happens to `git --quiet --
[<path>...]` too.  -a/--text allows us to include those files
regardless.

Signed-off-by: Zixuan James Li <zixuan@zulip.com>

Author: PIG208

tools/check

@@ -189,7 +189,8 @@ files_check() {
 check_no_changes() {
     local change_description="$1"; shift
 
-    if git diff --quiet -- "$@" \
+    # To include files excluded in `.gitattributes`, --text is necessary.
+    if git diff --text --quiet -- "$@" \
             && no_untracked_files "$@"; then
         return
     fi