Introduce `git__getenv` which is a UTF-8 aware `getenv` everywhere.
Make `cl_getenv` use this to keep consistent memory handling around
return values (free everywhere, as opposed to only some platforms).

Rename FALLBACK to UNSPECIFIED

When a refspec contains no rhs and thus won't cause an explicit update,
we skip all the logic, but that means that we don't update FETCH_HEAD
with it, which is what the implicit rhs is.

Add another bit of logic which puts those remote heads in the list of
updates so we put them into FETCH_HEAD.

Fallback describes the mechanism, while unspecified explains what the
user is thinking.

This is what we used to return in the settter and there's tests in
bindings which ask for this. There's no particular reason to stop doing
so.

A remote's URLs are now modified according to the url.*.insteadOf
and url.*.pushInsteadOf configurations. This allows a user to
replace URL prefixes by setting the corresponding keys. E.g.
"url.foo.insteadOf = bar" would replace the prefix "bar" with the
new prefix "foo".

These are updates, same as the rest, we should call this callback. As we
are using the callback, let's make sure to skip unnecessary updates.

When we moved from acting on the instance to acting on the
configuration, we dropped the validation of the passed refspec, which
can lead to writing an invalid refspec to the configuration. Bring that
validation back.

An anonymous remote is not configured and cannot therefore have
configured refspecs. Remove the parameter which adds this from the
constructor.