Signify Broken for Custom Perforce-Based VCS

A recent update to vim-signify breaks my configuration.

I am using vim-signify with a proprietary Perforce-Based VCS. It is mostly identical to Perforce, except with a different executable name. Very strange, I know...

Previously, I had everything working with the following values set:
```
let g:signify_vcs_list = [ 'perforce', 'hg', 'git' ]
let g:signify_vcs_cmds = { 'perforce': '/* Custom VCS Diff Command */' }
```

Now the output from `:SignifyDebug` shows the error `Not a git repository`.

Maybe this is an instance of RTFM, but nothing has jumped out at me yet. I suspect this is caused by some of the recent async/detection changes. Since my scenario/configuration is so unique, I will spend some time debugging the issue.

I read through Issue #319, and is sounds similar but unrelated. As you suggested there, I will revert to the legacy branch for now.

Start with :SignifyDebug and the recent async/detection changes, comparing the documented configuration with the legacy branch behavior. Reproduce the custom Perforce-based VCS setup and verify that detection uses the configured executable rather than falling through to Git; done means :SignifyDebug reports the custom VCS without the repository error.

Written by the indexing model from the issue text.