Figure out performance hot spots

On https://github.com/gentoo/gentoo rustygitprompt takes 0.5s. Not sure if this can be improved and which operation is the most expensive one. Some profiling might help.

```
[jule@localhost:~/git/gentoo]
$ time rustygitprompt 
🅼
real	0m0,585s
user	0m0,336s
sys	0m0,247s
```

The upstream https://github.com/git/git/blob/master/contrib/completion/git-prompt.sh git prompt is instantaneous.