If the user does not pass any refspecs to push, try to use those
configured via the configuration or via add_push().

We have the step-by-step method in the initialization function as we
want to remove references based on the list of references which are
already there, and we can use the convenience function for testing the
main push.

This function, similar in style to git_remote_fetch(), performs all the
steps required for a push, with a similar interface.

The remote callbacks struct has learnt about the push callbacks, letting
us set the callbacks a single time instead of setting some in the remote
and some in the push operation.

Refactor fetchhead

remote: rename _load() to _lookup()

odb: hardcode the empty blob and tree

git hardocodes these as objects which exist regardless of whether they
are in the odb and uses them in the shell interface as a way of
expressing the lack of a blob or tree for one side of e.g. a diff.

In the library we use each language's natural way of declaring a lack of
value which makes a workaround like this unnecessary. Since git uses it,
it does however mean each shell application would need to perform this
check themselves.

This makes it common work across a range of applications and an issue
with compatibility with git, which fits right into what the library aims
to provide.

Thus we introduce the hard-coded empty blob and tree in the odb
frontend. These hard-coded objects are checked for before going to the
backends, but after the cache check, which means the second time they're
used, they will be treated as normal cached objects instead of creating
new ones.

If the remote is anonymous, then we cannot check for any configuration,
as there is no name. Check for this before we try to use the name, which
may be a NULL pointer.

This fixes #2697.

This gets the value from branch.<foo>.remote.