ci: use local git mirror as clone reference

Eliminates Codeberg git clone rate limiting. The mirror at
/git-mirrors/harb.git (synced every 2 min) provides objects locally,
so the clone step only fetches deltas from Codeberg.

Volume mounted via WOODPECKER_BACKEND_DOCKER_VOLUMES.

.woodpecker/ci.yml

@@ -10,6 +10,7 @@ clone:
     image: woodpeckerci/plugin-git
     settings:
       depth: 50
+      reference: /git-mirrors/harb.git
       netrc_machine: codeberg.org
       netrc_username: johba
       netrc_password:

.woodpecker/e2e.yml

@@ -20,6 +20,7 @@ clone:
     image: woodpeckerci/plugin-git
     settings:
       depth: 50
+      reference: /git-mirrors/harb.git
       netrc_machine: codeberg.org
       netrc_username: johba
       netrc_password: