Your expect version changes behaviour: it does the allocation and string formatting unconditionally even if `have_dot_git(&cwd)` is `Some`. It is probably not a problem for this since the other operations are significantly more expensive, but it can be a gotcha if used inside a loop.
You're right, that's what I get for doing this fast. In that case I'd say
let dot_git_dir = have_dot_git(&cwd).unwrap_or_else(|| panic!("{} does not appear to have a controlling .git directory; you are not in a git repository!", cwd.display()));
