Support multiple exclude dirs

Now `exclude` field in the GitAction yaml only supports one directory to be excluded. Typical scenerios need to support multiple directories, e.g., `.gitaction`, `.git`.

The backend go-lang-in-toto supports multiple `-e` parameters, so how about changing `exclude` into a array ?

Start by locating where the GitAction YAML `exclude` value is parsed and where exclusions are passed to the in-toto backend. Confirm how multiple directories should map to repeated `-e` parameters, then verify that configurations containing `.gitaction` and `.git` both exclude the requested directories.

Written by the indexing model from the issue text.