test server: emit HEAD symref-target under v2 ls-refs symrefs

The v2 ls-refs handler ignored the "symrefs" argument and never emitted HEAD with a symref-target attribute, so source-side HEAD info captured via v2 was always empty in tests. Real git emits

when the client requests "symrefs" and a ref-prefix would include HEAD (or no prefixes are given). Add that to the test server with two small helpers: coversHead checks prefix coverage, lsRefsHeadLine formats the pkt-line by resolving HEAD's symref target through the repo.

Foundation for upcoming work on surfacing source HEAD.