script/ceph-backport: only fetch the merge sha1

Rather than doing a full fetch every time.

Signed-off-by: Patrick Donnelly <[email protected]>

Author: batrick

src/script/ceph-backport.sh

@@ -292,7 +292,7 @@ function cherry_pick_phase {
         git checkout -b "$local_branch" FETCH_HEAD
     fi
 
-    git fetch "$upstream_remote" "pull/$original_pr/head:pr-$original_pr"
+    git fetch "$upstream_remote" "$merge_commit_sha"
 
     set +x
     maybe_restore_set_x