fix: open and stage entries with no hunks, and surface git's own errors

[undont]

Summary

• open entries containing no hunks (chmod, empty new file, trailing-newline-only edit, rename, moved submodule pointer) on a diff naming the reason, instead of a panel row that did nothing and said "no changes" on every try
• name a submodule pointer move for what it is rather than reporting it as empty
• stage whole-file entries where s/u say they will: binary files, those zero-hunk changes, and a new file opened from a split's left column previously stepped to another file instead
• report git's own error when a listing fails, so a mistyped revspec surfaces fatal: bad revision rather than "no changes for this source"; covers the panel, :Differ log and :Differ log <range>
• count untracked lines from disk once and cache until mtime or size moves; every build and refresh used to re-read them, and any non-binary file containing a carriage return also left a loose object in .git/objects each time
• bound the :Differ pr checkout fetch (M.fetch_timeout_ms, 20s) and disable credential prompts (it froze nvim for as long as a slow remote took, and hung for good on a prompt), and hand back a message when git fails to spawn instead of a Lua error

Test plan

• make check clean (fmt, luacheck, golangci-lint, lua_ls, go vet, demo build)
• lua unit 387/0, headless-nvim 480/0
• go tests pass