Add optional refreshing in the fake backend, and count the number of
refresh calls if enabled.

The majority of functions are named `from_something` (with an
underscore) instead of `fromsomething`.  Update the blob functions for
consistency with the rest of the library.

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.