Record the default branch summary when a push lands

Record the default branch summary when a push lands

The landing card showed the default branch tip and the language it is
mostly made of by running `symbolic-ref`, `rev-parse`, `log -1` and
`ls-tree -r` per repository on every home page read. Those facts change
only when a push moves the branch, so the push now records them.

- `repos` carries head_sha, head_subject, head_author, head_when,
  language and lang_class, and `Index.Entry` returns them with the rest of
  the row.
- `Store.RefreshHead` recomputes the summary and skips the tree walk when
  the recorded tip is still the branch tip, so a metadata or tag push
  costs one rev-parse. It runs from post-receive, from the rebuild, and
  after the default branch changes in settings.
- The language mapping moved to `internal/lang`, shared by the web pages
  and the summary; `fileClass` and `languageFor` are now one table.
- The home cards read the head, language, counts, ACL and newest event
  time from the index. The home page starts no git process at all; the
  all-git card builder stays as the fallback for a Web without an index.

Load harness at concurrency 4 with three repositories: home goes from 243
to 749 req/s, p50 from 16 ms to 5 ms, and the server runs no git child
process for those requests.