Remove is-true function
Remove is-true function

that was only being used in the `git` and `git-info` modules to check
the output of `git rev-parse --is-inside-work-tree`. The code depending
on this function was already changed.

Closes #163

joshuarubin/dotfiles,Eriner/zim,joshuarubin/dotfiles,joshuarubin/dotfiles,joshuarubin/dotfiles