Problem: running :e on a :Gdiff buffer cleared all content because
diffs:// buffers had no BufReadCmd handler. Neovim tried to read the
buffer name as a file path, found nothing on disk, and emptied the
buffer. This affected all three buffer creation paths (gdiff,
gdiff_file, gdiff_section).
Solution: register a BufReadCmd autocmd for diffs://* that parses the
URL and regenerates diff content from git. Change buffer options from
nofile/wipe to nowrite/delete (matching fugitive's approach) so
buffer-local autocmds and variables survive across unload/reload
cycles. Store old filepath as buffer variable for rename support.
