sync_templates_and_ci: use rsync (#24341)

* sync_templates_and_ci: use rsync

rsync allows us to delete files present at the destination but
not the source.

Also split arguments into multiple lines for readability/maintainability.

* Collapse rsync argument list

developer/bin/sync_templates_and_ci

@@ -31,7 +31,7 @@ function copy_templates_and_ci {
   local repo_name
 
   repo_name="${1}"
-  cp -R "${main_repo_dir}/.github" "${main_repo_dir}/.travis.yml" "${main_repo_dir}/ci" '.'
+  rsync --archive --delete "${main_repo_dir}"/{.github,.travis.yml,ci} '.'
   /usr/bin/sed -i '' -E "s:homebrew-cask/(pulls|issues):${repo_name}/\1:" '.github/PULL_REQUEST_TEMPLATE.md' # PULL_REQUEST_TEMPLATE has repo-specific links
 }