jobs/gerrit…-dahdi: don't use multiple scms plugin

Don't use the deprecated plugin. Let jenkins clone the big linux
repository and clone the code from gerrit in the shell section of the
script. Since we can't directly clone $GERRIT_REFSPEC, create an empty
git repository and fetch it.

Related: OS#5763
Change-Id: I52314f55e7e55382b4e8fcd63d3aba306b401447

jobs/gerrit-verifications-dahdi.yml

@@ -29,23 +29,9 @@
       <b>Auto-generated using Jenkins Job Builder. DO NOT EDIT MANUALLY!</b>
 
     scm:
-      - git:
-         basedir: 'dahdi-linux'
-         url: 'https://gerrit.osmocom.org/dahdi-linux'
-         credentials-id: d5eda5e9-b59d-44ba-88d2-43473cb6e42d
-         branches:
-           - $GERRIT_BRANCH
-         refspec: $GERRIT_REFSPEC
-         choosing-strategy: gerrit
-         # When the gerrit git repository is configured to wipe-workspace, the
-         # linux.git clone gets removed as well. Cloning the linux repo takes a
-         # long time, so only clean both repos, don't remove them.
-         wipe-workspace: false
-         clean:
-          before: true
-         skip-tag: true
-         submodule:
-           recursive: true
+      # Let jenkins clone the big linux repository and keep it between builds
+      # (wipe-workspace is false). Clone dahdi-linux with code from gerrit in
+      # the shell part below.
       - git:
          basedir: 'linux'
          url: 'https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git'
@@ -80,6 +66,17 @@
 
     builders:
       - shell: |
+          rm -rf dahdi-linux
+          git init dahdi-linux
+          cd dahdi-linux
+          git config advice.detachedHead false
+          git fetch \
+            --depth=1 \
+            https://gerrit.osmocom.org/dahdi-linux \
+            "$GERRIT_REFSPEC"
+          git checkout FETCH_HEAD
+          cd ..
+
           docker run \
             --rm=true \
             -e "KSRC=/linux" \