drm_hwcomposer: Try to fix clang-format pipeline

The clang-format pipeline generates false negatives
sometimes, with:

$ git diff -U0 --no-color master...HEAD | clang-format-diff-3.5 -p 1 -style=file > format-fixup.patch
fatal: ambiguous argument 'master...HEAD': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:

as seen here:
https://gitlab.freedesktop.org/john.stultz/drm-hwcomposer/-/jobs/932

This tries to address the issue by explictly fetching
freedesktop/master and referencing FETHC_HEAD instead of master
which doesn't always exist.

Change-Id: I7a07d412eae164841427da90dfc7298697c5e783
Reviewed-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index b974875..43bce5f 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -10,7 +10,8 @@
 clang-format:
   stage: style
   script:
-    - git diff -U0 --no-color master...HEAD | clang-format-diff-3.5 -p 1 -style=file > format-fixup.patch
+    - git fetch https://gitlab.freedesktop.org/drm-hwcomposer/drm-hwcomposer.git
+    - git diff -U0 --no-color FETCH_HEAD...HEAD -- | clang-format-diff-3.5 -p 1 -style=file > format-fixup.patch
     - if [ -s format-fixup.patch ]; then cat format-fixup.patch && exit 1; fi
   artifacts:
     when: on_failure