Checking executable() is slow, avoid when possible

Checking executable() for every possible VCS is fairly slow at startup.
This change, combined with specifying vcs and difftool, shaves of
155 ms (half) of my startup time.

    let g:signify_vcs_list = ['git']
    let g:signify_difftool = 'diff'

Feel free to throw away this and implement it in another way.
I don't like that specifying signify_difftool to the same value as default improves performance.