remove redundant path modification for diff

the runtime directory is automatically added to the path; see `:h win32-path`; so `diff.exe` should be available.

this also "fixes" the error introduced with commit https://github.com/mhinz/vim-signify/commit/9a78f90e1aa2a656366217d1bcea70ed76b68309, since we're supplying only `diff` to the `--diff-command` now instead of something with spaces.