Rollup merge of rust-lang#88699 - Mark-Simulacrum:fixes-cherry-picker, r=pietroalbini

Remove extra unshallow from cherry-pick checker

This is already done by https://github.com/rust-lang/rust/blob/13db8440bbbe42870bc828d4ec3e965b38670277/src/ci/init_repo.sh#L32-L36 on the beta channel, and git throws an error if you attempt to unshallow an already non-shallow repository.

r? ```@pietroalbini```

Author: Manishearth

src/ci/scripts/verify-backported-commits.sh

@@ -18,14 +18,6 @@ verify_backported_commits_main() {
     exit 0
   fi
 
-  echo 'git: unshallowing the repository so we can check commits'
-  git fetch \
-    --no-tags \
-    --no-recurse-submodules \
-    --progress \
-    --prune \
-    --unshallow
-
   if [[ $ci_base_branch == "beta" ]]; then
     verify_cherries master "$BETA_LIMIT" \
       || exit 1