Update the method documentation for `ObjectDatabase.Write` to be
consistent with the other methods.

Test that we can `ObjectDatabase.Write` a blob.

Provide a mechanism to write a stream directly to an object database.

Allow consumers to add a blob to a `TreeDefinition` by specifying only
the object ID and mode.  This lets users build tree entries without
having to create a `Blob` object (and thus load the object itself)
which is beneficial for users building trees with large objects.

This is only useful for Blobs, since they do not need to be fetched from
the object database to be realized within the tree builder.  Users are
not able to add Trees by ID, since we would need to load them (and
cannot, since we are not guaranteed to be instantiated within the
context of a repository).

Include `TreeDefinition.Add` by Object ID.
Include `ObjectDatabase.Write` to write a stream.

Backports to 0.24.1

The subtransport tests create a new subtransport and tests that it can
talk to github.com.  GitHub recently required TLS 1.2 connections,
upgrade our test to build a TLS 1.2 connection to GitHub.

mono 4.8 on mac simply cannot cope with TLS 1.2; newer versions are not compatible with our travis ci setup.  give up and just use bitbucket.

Update libgit2 to v0.26.2

appveyor: maint branches aren't prerelease

 Update libgit2 to 0.26.3