Move to the `git_error` name in the internal API for error-related
functions.

Use the new object_type enumeration names within the codebase.

In commit 7ec7aa4a7 (odb: assert on logic errors when writing objects,
2018-02-01), the check for whether we are trying to overflowing the fake
stream buffer was changed from returning an error to raising an assert.
The conversion forgot though that the logic around `assert`s are
basically inverted. Previously, if the statement

    stream->written + len > steram->size

evaluated to true, we would return a `-1`. Now we are asserting that
this statement is true, and in case it is not we will raise an error. So
the conversion to the `assert` in fact changed the behaviour to the
complete opposite intention.

Fix the assert by inverting its condition again and add a regression
test.

Our mempack ODB backend has no test coverage at all right now. Add a
simple test suite to at least have some coverage of the most basic
operations on the ODB.

The null OID (hash with all zeroes) indicates a missing object in
upstream git and is thus not a valid object ID. Add defensive
measurements to avoid writing such a hash to the object database in the
very unlikely case where some data results in the null OID. Furthermore,
add shortcuts when reading the null OID from the ODB to avoid ever
returning an object when a faulty repository may contain the null OID.

Introduce a new test suite "odb::backend::simple", which utilizes the
fake backend to exercise the ODB abstraction layer. While such tests
already exist for the case where multiple backends are put together, no
direct testing for functionality with a single backend exist yet.

The fake backend currently implements all reading functions except for
the `exists_prefix` one. Implement it to enable further testing of the
ODB layer.

The `search_object` function takes the OID length as one of its
parameters, where its maximum length is `GIT_OID_HEXSZ`. The `exists`
function of the fake backend used `GIT_OID_RAWSZ` though, leading to
only the first half of the OID being used when finding the correct
object.

In order to be able to test the ODB prefix functions, we need to be able
to detect ambiguous prefixes in case multiple objects with the same
prefix exist in the fake ODB. Extend `search_object` to detect ambiguous
queries and have callers return its error code instead of always
returning `GIT_ENOTFOUND`.

Previous to pulling out and extending the fake backend, it was quite
cumbersome to write tests for very specific scenarios regarding
backends. But as we have made it more generic, it has become much easier
to do so. As such, this commit adds multiple tests for scenarios with
multiple backends for the ODB.

The changes also include a test for a very targeted scenario. When one
backend found a matching object via `read_prefix`, but the last backend
returns `GIT_ENOTFOUND` and when object hash verification is turned off,
we fail to reset the error code to `GIT_OK`. This causes us to segfault
later on, when doing a double-free on the returned object.