Add a git pull check
fix quoting and simplify upstream reference check

Closes #19
Use `command` for git and silence `git fetch`
fix mangled prompts. closes #30

1. error message printed when no configured upstream branch
2. 256 color code used in precmd doesn't work on my terminal
3. preexec hook mangles output when the command itself contains escape sequences
4. cursor position isn't restored correctly when you have already began typing something before the async command executes