When attempting to update a reference on a remote during push, and the
reference on the remote refers to a commit that does not exist locally,
then we should report a more clear error message.

This describes their purpose better, as we now initialize ssl and some
other global stuff in there. Calling the init function is not something
which has been optional for a while now.

Refactor fetchhead

remote: rename _load() to _lookup()

odb: hardcode the empty blob and tree

This is an ugly chunk of code, so let's put it into its own function.

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.