scripts/refresh-submodules.sh: use the correct sha1 in title

0d4ffe1d69 introduced a regression where
it used the sha1 of the local "master" branch instead of the remote's
"master" branch in the title of the commit message.

in this change, let's use the origin/${branch}'s sha1 in the title.

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

Closes #13974

scripts/refresh-submodules.sh

@@ -36,7 +36,7 @@ for ent in "${@:-${submodules[@]}}"; do
     fi
     # collect the summary
     head_ref=$(git rev-parse --short=8 HEAD)
-    branch_ref=$(git rev-parse --short=8 ${branch})
+    branch_ref=$(git rev-parse --short=8 origin/${branch})
     count=$(git rev-list --no-merges --count HEAD..${bump_to})
     # create a summary using the output format of "git submodule summary"
     SUMMARY="