Preventing find_command() to find cmds without a './' prefix
This commit fixes #684. The original implementation of find_command()
will find cmd inside the working directory even without
the prefix './'. However, those cmds would have not been visible in
the auto completion. This commit prevents this from happening.
The problem can be traced back to vfs_absolutize() which treats paths
without the './' prefix as relative paths. The changes made unsures
that only relative or absolute pathes will be passed to vfs_absolutize()
