Add GitBash

[abhidg]

Required for UNIX Shell lessons and git lessons.

[ethanwhite]

Good call. What's the preferred source for Git for macOS these days?

[abhidg]

The preferred method seems to be XCode command line tools or brew install git, neither of which lend itself to easy offline mirroring. The binary git download https://git-scm.com/download/mac is outdated.

[ethanwhite]

Thanks. In that case do we want to scale this issue back to GitBash for now?