drm_hwcomposer: Try to fix clang-format pipeline

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>