remote: allow overriding the refspecs for download and fetch

With opportunistic ref updates, git has introduced the concept of having
base refspecs *and* refspecs that are active for a particular fetch.

Let's start by letting the user override the refspecs for download.
remote: use active refspec override in the tests

This lets us test this bit as well as getting closer to what they were
trying to do.
remote: don't DWIM refspecs unnecessarily

We can only DWIM when we've connected to the remote and have the list of
the remote's references. Adding or setting the refspecs should not
trigger an attempt to DWIM the refspecs as we typically cannot do it,
and even if we did, we would not use them for the current fetch.
remote: store passive refspecs

The configured/base fetch refspecs need to be taken into account in
order to implement opportunistic remote-tracking branch updates. DWIM
them and store them in the struct, but don't do anything with them yet.
remote: implement opportunistic remote-tracking branch updates

When a list of refspecs is passed to fetch (what git would consider
refspec passed on the command-line), we not only need to perform the
updates described in that refspec, but also update the remote-tracking
branch of the fetched remote heads according to the remote's configured
refspecs.

These "fetches" are not however to be written to FETCH_HEAD as they
would be duplicate data, and it's not what the user asked for.
Merge pull request #2462 from libgit2/cmn/remote-fetch-refs

Implement opportunistic ref updates