Prefer String over Path/OsString

[robertdfrench]

We don't need Path anymore, because @calebwherry found that we don't need the Path construction stuff that I assumed we needed on Windows. I also didn't realize that String implements a trait called AsRef<OsStr>, meaning that it can be passed as a reference to std::process::Command::new. So that will bring the number of string types in this code from 3 down to 1!

_Originally posted by @robertdfrench in https://github.com/robertdfrench/git-pr/pull/1#discussion_r725710107_

[robertdfrench]

• String implements AsRef<OsStr>: https://doc.rust-lang.org/std/string/struct.String.html#impl-AsRef%3COsStr%3E
• Definition of AsRef trait: https://doc.rust-lang.org/std/convert/trait.AsRef.html
• Interface of Command::new: https://doc.rust-lang.org/std/process/struct.Command.html#method.new