Previously, we were using the relative path to extract the search
string. For the current directory, the returned relative path was equal
to `""`. This is why the current directory did not show in the search
results (see #81).
This commit also changes the way that `--full-path` works, which was
previously working on relative paths. It seems more useful to search the
absolute path, though. Otherwise, search results could change just by
calling fd (with a given search path) from a different directory.
