This flips rename detection around so instead of creating a
forward mapping from deltas to possible rename targets, instead
it creates a reverse mapping, looking at possible targets and
trying to find a source that they could have been renamed or
copied from. This is important because each output can only
have a single source, but a given source could map to multiple
outputs (in the form of COPIED records).
Additionally, this makes a couple of tweaks to the public rename
detection APIs, mostly renaming a couple of options that control
the behavior to make more sense and to be more like core Git.
I walked through the tests looking at the exact results and
updated the expectations based on what I saw. The new code is
different from the old because it cannot give some nonsense
results (like A was renamed to both B and C) which were part of
the outputs previously.
I frequently want to the the first N digits of an OID formatted
as a string and I'd like it to be efficient. This function makes
that easy and I could rewrite the OID formatters in terms of it.
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.
Previously a NULL oid was handled like an empty buffer and
returned a status empty string. This makes git_oid_tostr()
set the output buffer to the empty string instead.
