Summary:
When using this backend in a directory that is not a git repository, it will
search though all the file. This is mostly a problem when using an alternative
source control like sapling.
When using this backend neovim will just hang until the rg command is done.
When searching node_modules you can imagine how long that takes. This does not
actually fix the issue however, it does make it search though less files.
Test Plan:
This has been used manually on a sapling repo. I have also added
`--no-require-git` to most of the rg defaults in other applications.
`rg` will now be used over `ag` if it is available. This is because `rg`
is a faster alternative reduces lag when searching larger codebases.
`ag` is also now conditionally loaded if the command is available.
