CI: move whitespace check to external script

Without this measure, a failed check is no longer detected.  This seems
to be a bug in gitlab CI.

.gitlab-ci.yml

@@ -36,8 +36,7 @@ build package:
 
 check whitespace style:
   stage: build
-  script:
-    - "! for f in `git ls-files | grep -v \\.diff$`; do file $f | grep -q ' text' || continue; grep -q '[[:blank:]]$' $f && echo $f: trailing whitespace; tail -n1 $f | read -r _ || echo $f: no newline at end of file; tail -n1 $f | grep -q '^$' && echo $f: empty line at end of file; done | grep . >&2"
+  script: ./check_whitespace
   allow_failure: true
 
 check for dependencies installed:

check_whitespace

+#!/bin/sh
+
+! for f in $(git ls-files | grep -v \\.diff$); do
+    file $f | grep -q ' text' || continue
+    grep -q '[[:blank:]]$' $f && echo $f: trailing whitespace
+    tail -n1 $f | read -r _ || echo $f: no newline at end of file
+    tail -n1 $f | grep -q '^$' && echo $f: empty line at end of file
+done | grep . >&2