checks for exa command & the --git option

Same problem (https://github.com/zimfw/exa/pull/3) fix here:

https://github.com/zimfw/exa/commit/6c9fce30e31739fe93898d564447806a99a7a977#diff-4b34a27687475c10b944d7952193a477697be078847593c216ab6c40149112e7