Exit when directory doesn't contain a .git dir

- this prevents the following error, when initializing in a non-git directory

/usr/local/bin/gitcrypt: line 96: .git/info/attributes: No such file or directory
/usr/local/bin/gitcrypt: line 97: .git/info/attributes: No such file or directory
/usr/local/bin/gitcrypt: line 98: .git/info/attributes: No such file or directory
error: could not lock config file .git/config: No such file or directory
error: could not lock config file .git/config: No such file or directory
error: could not lock config file .git/config: No such file or directory
error: could not lock config file .git/config: No such file or directory
error: could not lock config file .git/config: No such file or directory
error: could not lock config file .git/config: No such file or directory