Reduce code complexity

[nicolas377]

A lot of our code isn't well documented, and it's hard to understand.
Here's some of what we need to do.

• Add comments
• Simplify actual code
• refactor: use SimpleGit.checkIsRepo() instead of shell commands #67
• refactor: use a simpler implementation for isGitInstalled() #64

We can gradually do this over time, possibly for v1