[tummetott]

The diff_buf_read and diff_buf_win_enter hooks only report the window's layout position (symbol, layout_name).

This PR extends the hook context with:

• rev: type ("LOCAL", "COMMIT", "STAGE" or "CUSTOM"), commit, stage and tracks_head
• path: the file path relative to toplevel
• toplevel: the repository root
• adapter: "git", "hg", "jj" or "p4" (nil outside a repository)

The DiffviewDiffBufRead and DiffviewDiffBufWinEnter User autocmds carry the same fields in data, together with bufnr and winid. The existing fields are unchanged.

This is especially useful for winbar and statusline plugins, which can now label each diff window with the revision it shows (LOCAL, INDEX, HEAD, a commit hash) directly from the hook:

hooks = {
  diff_buf_win_enter = function(bufnr, winid, ctx)
    vim.w[winid].diffview_ctx = ctx
  end,
}

A statusline component then reads vim.w.diffview_ctx when it draws.