CI: add CI_CLEAN_REPOS variable to allow cleaning the repo reference cache

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>

.gitlab-ci.yml

@@ -58,7 +58,8 @@ stages:
       - $CI_PROJECT_DIR/work/build/tmp/work*/**/testimage/*
 
 #
-# Prep stage, update repositories once
+# Prep stage, update repositories once.
+# Set the CI variable CI_CLEAN_REPOS=1 to refetch the respositories from scratch
 #
 update-repos:
   extends: .setup

ci/update-repos

@@ -4,6 +4,7 @@
 
 import sys
 import os
+import shutil
 import subprocess
 import pathlib
 
@@ -34,9 +35,14 @@ if __name__ == "__main__":
 
     for repo in repositories:
         repodir = base_repodir / repo_shortname(repo)
+
+        if "CI_CLEAN_REPOS" in os.environ:
+            print("Cleaning %s..." % repo)
+            shutil.rmtree(repodir, ignore_errors=True)
+
         if repodir.exists():
             print("Updating %s..." % repo)
-            subprocess.run(["git", "-C", repodir, "fetch"], check=True)
+            subprocess.run(["git", "-C", repodir, "-c", "gc.autoDetach=false", "fetch"], check=True)
         else:
             print("Cloning %s..." % repo)
             subprocess.run(["git", "clone", "--bare", repo, repodir], check=True)