PySVN and Subvertpy had very different ways of getting history logs from

a repository. Both had functions that accepted paths and revision

ranges, but they had different defaults for these parameters and

represented the results in different ways.
This adds a get_log() function for the SVN client backends that return

entries in consistent ways. It'll soon be used to centralize some of the

logic that is currently duplicated between the backends.

Made use of these in an upcoming change. This particular change isn't unit tested,

but the changes making use of this data are.