To support multiple different reference backend implementations,
Git introduced a "refStorage" extension that stores the reference
storage format a Git client should try to use.
Wire up the logic to read this new extension when we open a repository
from disk. For now, only the "files" backend is supported by us. When
trying to open a repository that has a refstorage format that we don't
understand we now error out.
There are two functions that create a new repository that doesn't really
have references. While those are mostly non-functional when it comes to
references, we do expect that you can access the refdb, even if it's not
yielding any refs. For now we mark those to use the "files" backend, so
that the status quo is retained. Eventually though it might not be the
worst idea to introduce an explicit "in-memory" reference database. But
that is outside the scope of this patch series.
This moves most of the refdb stuff over to the include/git2/sys
directory, with some minor shifts in function organization.
While I was making the necessary updates, I also removed the
trailing whitespace in a few files that I modified just because I
was there and it was bugging me.
