acai: replace `depth` option with `ref`

[MartinHelmut]

Currently the scanner (packages/acai/src/scanner.js) uses as option depth to shorten the output. This should be replaced with ref getting a commit or version reference. Possible configurations are:

// From one version to another
{ ref: 'v1.0.0..v2.0.0' }
// From a specific commit hash
{ ref: '4f7abfd' }
// is the same as:
{ ref: '4f7abfd..HEAD' }

This is also compatible with Mercurial (#4)