By the time we recognise this as an ambiguous id, the object's data
has been loaded into memory. Free it when returning EABMIGUOUS.

This is based on 24634c6f.

This also corrects an issue with error codes being mixed up with the
number of found objects.

The test coverage for ambiguous OIDs was pretty thin.  This adds
a bunch of new objects both in packs, across packs, and loose that
match to 8 characters so that we can test various cases of
ambiguous lookups.

This is a significant reorganization of the diff code to break it
into a set of more clearly distinct files and to document the new
organization.  Hopefully this will make the diff code easier to
understand and to extend.

This adds a new `git_diff_driver` object that looks of diff driver
information from the attributes and the config so that things like
function content in diff headers can be provided.  The full driver
spec is not implemented in the commit - this is focused on the
reorganization of the code and putting the driver hooks in place.

This also removes a few #includes from src/repository.h that were
overbroad, but as a result required extra #includes in a variety
of places since including src/repository.h no longer results in
pulling in the whole world.

This moves some of the odb_backend stuff that is related to the
internals of an odb_backend implementation into include/git2/sys.

Some of the stuff related to streaming I left in include/git2
because it seemed like it would be reasonably needed by a normal
user who wanted to stream objects into and out of the ODB.

Also, I added APIs for traversing the list of backends so that
some of the tests would not need to access ODB internals.

This makes the first OID param of the ODB callback a const pointer
and also propogates that change all the way to the backends.

Without this change, any failed assertion in the second (or a later) test
inside a test suite has a chance of double deleting memory, resulting in
a heap corruption. See #1096 for details.

This leaves alone the test cases where we "just" use cl_git_sandbox_init()
and cl_git_sandbox_cleanup(). These methods already take good care to not
double delete a repository.

Fixes #1096

The maximum depth is 5, like in git

Fixes #921.

This updates all the `foreach()` type functions across the library
that take callbacks from the user to have a consistent behavior.
The rules are:

* A callback terminates the loop by returning any non-zero value
* Once the callback returns non-zero, it will not be called again
  (i.e. the loop stops all iteration regardless of state)
* If the callback returns non-zero, the parent fn returns GIT_EUSER
* Although the parent returns GIT_EUSER, no error will be set in
  the library and `giterr_last()` will return NULL if called.

This commit makes those changes across the library and adds tests
for most of the iteration APIs to make sure that they follow the
above rules.

git_odb_backend_one_packfile() allows us to create an ODB backend out
of an .idx file.

Some objects were added in another PR

Go through each backend and list every objects that exists in
them. This allows fsck-like uses.

Clay is the name of a programming language on the makings, and we want
to avoid confusions. Sorry for the huge diff!