Add a git client

[AtkinsSJ]

Builds on #413.

A git app, that can be run inside Puter's terminal.

This is limited to the following commands, which are all implemented to some extent:

• git help
• git version
• git init
• git status
• git add
• git commit
• git log
• git config
• git show

It still lacks:

• Branches
• Tags
• Anything that interacts with a remote server (eg cloning, pulling, or pushing)
• Rebasing
• Stashes
• A lot of functionality with those implemented commands. To say that git is very configurable is an understatement. 😅

[ghost]

Cant wait... I want it, I want it, I want it, I want it, I want it, I want it, I want it, I want it, I want it, I want it, I want it, I want it, I want it, I want it,, I want it, I JUST WANT IT

[triptych]

Yes, this is by far my number 1 request for puter.

[AtkinsSJ]

I think this is in a decent enough state to be worth reviewing.

A couple of things that have come up:

• It'd be nice to have a better API for stdin/stdout/stderr, that just take strings, and all the encoding is kept internal
• Passing an exit code to puter.exit() would be convenient - Add exit status codes to puter.exit(), and an exit builtin to Phoenix #421
• Being able to launch a background app from the start menu is a bit odd. For git, I don't think you can usefully do anything, but for some console apps like a python interpreter, you'd want a way of forcing it to have a parent terminal and shell.

There's some further work in #418 but CORS is not my friend so that has not gone anywhere yet.