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.

We go through the different backends in order, so it's not an error if
at least one of the backends has the data we want.

There were a couple of "init_opts()" functions a few more cases
of structure initialization that I somehow missed.

Fix git_odb_short_id and git_odb_exists_prefix bugs

Fixes issue #2196

The git_odb_exists_prefix API was not dealing correctly when a
later backend returned GIT_ENOTFOUND even if an earlier backend
had found the object.

Additionally, the unit tests were not properly exercising the API
and had a couple mistakes in checking the results.

Lastly, since the backends are not expected to behavior correctly
unless all bytes of the short id are zero except for the prefix,
this makes the ODB prefix APIs explicitly clear out the extra
bytes so the user doesn't have to be as careful.

The basic structure of each function is courtesy of arrbee.

Add ODB API to check for existence by prefix and object id shortener