branch: only perform HEAD check for local branches

When assembling the ref-filter format to show "git branch" output, we put the "%(if)%(HEAD)" conditional at the start of the overall format. But there's no point in checking whether a remote branch matches HEAD, as it never will. The check should go inside the local conditional; we assemble that format inside the "local" strbuf. By itself, this is just a minor optimization. But in a future patch, we'll need this refactoring to fix local-branch coloring. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>