branch: only perform HEAD check for local branches

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>