fix: Fix StoredUserConfig not escaping control characters

fix: Fix StoredUserConfig not escaping control characters

The `StoredUserConfig` only escaped the escape character, i.e. backslash.
But it does not escape control characters like tab or newline. This
introduces a vulnerability where an attacker can create new entries
in their user account and create new accounts.
In addition, other characters are also not properly handled. Field values
with a comment character need to be quoted. This only happens for the
`#` character and only when the value starts with it. Also the quote
is note escaped in values.

This change completely rewrites the `escape` method of `StoredUserConfig`.
It takes care of properly escaping characters that need escaping for the
git configuration file format.

This fixes #1410