GitLab

If the repository reference directory gets corrupted it's not easy to
wipe it, so add a variable CI_CLEAN_REPOS that if set in the pipeline
will clean the clones and re-fetch them.

Also, stop the fetch from detaching during the garbage collection, just
in case it was a long-running GC that got killed that caused the
corruption in the first place.
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>