We very often expect success, and we sometimes want to get the stdout,
too. Let's add a convenience function for that. It saves a lot of
lines of code.
I thought that `std::fs::canonicalize()` expanded "~", but it doesn't
seem to do that, which caused #131. Git seems to do the expansion
itself, so we probably also should. More importantly
`std::fs::canonicalize()` crashes when the file doesn't exist. The
manual expansion we do now does not.
Closes#131.
It probably doesn't make sense to respect Git's `core.excludesFile`
config when not running in a Git-backed repo, but we also already
respect `.gitignore` files in the working copy regardless of backend,
so at least it's consistent with that. We can revisit it when the
native backend becomes a reasonable choice.
Closes#87.
