Use minimal file paths.

Since we start all external commands by cd'ing to the current file's
directory, we can use the file's name instead of its full path.

The exception is `git show :FILE` where we must use a path relative to
the repo root for maximum git compatibility.  It turns out `git
rev-parse` takes a `--show-prefix` argument which makes our relative
path calculation far simpler.

Minimising files paths has these benefits:

- Easier to inspect the generated commands.
- Less opportunity for escaping problems.
- Eliminates possible mismatches betwen absolute paths generated by git
  and absolute paths generated by Vim (crops up with msys/msys2 on
  Windows).

Thanks to @suxpert for helping with this.