emacs helm-xref: full file paths

https://github.com/brotzeit/helm-xref/issues/19#issuecomment-1145914982

To use those long path function, use:

M-x set-variable helm-xref-candidate-formatting-function helm-xref-format-candidate-full-path

or put this in your .emacs file:

(setq helm-xref-candidate-formatting-function 'helm-xref-format-candidate-full-path)