Implement SVNClient.parse_revision_spec and add tests.
This implements revision parsing for the SVNClient. Like with the current

implementation for Git, you can choose to pass zero, one, or two arguments.
In the case of zero, it returns a special internal string that I'm planning on

using as a notice for the different 'svn diff' invocation syntax that we have

to use for diffing the working directory.
One argument can be either a numeric/symbolic revision, a revision span, or a

changelist name. If the argument has a : in it, it's split and treated as two

arguments. If not, we use 'svn log' to determine if it's a real revision, and

if that fails, 'svn status --cl' to check if it's a changelist. If it's none of

the above, we raise an exception.
With two arguments, the two arguments must be revisions of some sort. Like with

the single argument, we use 'svn log' to turn these into actual numbers.
Unlike the Git implementation, this is not actually used for the diff

generation yet.

Ran unit tests