"Connection data" is an imprecise and largely incorrect name; these
structures are actually parsed URLs.  Provide a parser that takes a URL
string and produces a URL structure (if it is valid).

Separate the HTTP redirect handling logic from URL parsing, keeping a
`gitno_connection_data_handle_redirect` whose only job is redirect
handling logic and does not parse URLs itself.

When we're dealing with proxy addresses, we only want a hostname and
port, and the user would not provide a path, so make it optional so we
can use this same function to parse git as well as proxy URLs.

In order to connect to a remote server, we need to provide a path to the
repository we're interested in. Consider the lack of path in the url an
error.