Get the number of unstaged changes (lines) for Git

This function provides a way to display the number of unstaged changes when the working directory is part of a Git project (e.g. `+12/-3`). If there is no such unstage change, nothing is printed.

One just needs to add `$(git_changes_info)` in a theme, and `ZSH_THEME_GIT_CHANGES_PREFIX/SUFFIX` can be used to modify the display.

This is an idea I got from nojhan's [liquidprompt](https://github.com/nojhan/liquidprompt).