Revert "fix spelling in git.fish"

"NUL" is the proper terminology

This reverts commit 77c9823dc4459305873907ff96ff314dc915c19a.
Revert "fix spelling in hg.fish"

"NUL" is the proper terminology

This reverts commit 9083adc0f89ffa076b19f9676baf42209bd5af4c.