Use relative paths when calling Git on Windows

On Unix, use absolute paths:
When a file has been opened via a symlink, 'file-relative-name' will
fail because 'git-gutter:root-directory' (which points to the original
directory) and 'buffer-file-name' (which holds the link alias) don't
match up.

(On Windows, 'buffer-file-name' always returns the original file
name, so 'file-relative-name' is safe to use.)

Fixes #1