Make relative path resolve to afn_proj_root

Original plugin design was to only resolve absolute paths (starting with
a /) relative to the afn_proj_root. In some C++ code bases though,
include paths are meant to be relative to the repository root.
Automatically detect the afn_proj_root path

Use git to detect the repository root of the currently open file. If
file is not in a git repo, nothing will change. If it is, we'll use git
to detect root of the repo once per view and update afn_proj_root to
reflect that.