Escape messages in 'zbg stash'

[chshersh]

Currently, messages for the zbg stash command are not escaped. This can result in weird behaviour.

https://github.com/chshersh/zbg/blob/42bdb5c0df7af5f2608ff100003368a0e9ac3587/lib/git.ml#L125

Probably, the quote_command function from Unix should be used.

• [ ] Escape the message argument
• [ ] Write tests