git package is a bit of a mess. Not only does it define the interfaces
and types that the rest of the application should use, various
implementations are also defined in the package, most notably the
CommandFactory. This makes it difficult for other packages to actually
rely on the types in the Git package. While they may want to import a
type like `git.ObjectID`, the implementation in the package may easily
lead to cyclic dependencies.

This is the case for example with the storage package where we'd like
to define a transaction interface that the rest of the application
could import. This is currently not possible as the transaction interface
uses `git.ObjectID` which leads to a cyclic import as the command factory
implementation requires `storage.Locator`. This has lead to defining the
Transaction interface in the storagectx package as a workaround.

We'd like to now define the storage layer's interface in the storage
package, and this includes the Transaction. This commit ...