ci: Move style-check-diff commands out to a separate sh file

This doesn’t change how they run, but does split the code out a bit and
mean we can interleave it with comments. Should make it a little less
vile.

Suggested by Emmanuele Bassi; see !1252.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

.gitlab-ci.yml

@@ -26,9 +26,7 @@ style-check-diff:
   stage: style-check
   allow_failure: true
   script:
-    - git remote add upstream https://gitlab.gnome.org/GNOME/glib.git
-    - git fetch upstream
-    - git diff -U0 --no-color $(diff --old-line-format='' --new-line-format='' <(git rev-list --first-parent upstream/${CI_MERGE_REQUEST_TARGET_BRANCH_NAME:-${CI_DEFAULT_BRANCH}}) <(git rev-list --first-parent HEAD) | head -1) | ./clang-format-diff.py -binary "clang-format-7" -p1
+    - .gitlab-ci/run-style-check-diff.sh
 
 fedora-x86_64:
   extends: .only-default