_ComputeRepositoryURL requires more info than IProperties makes available

[dustin]

In 0.8.4, the class of the object passed to getRenderingFor methods was somewhat vague - a Build in some cases, BuildStatus in others.

I changed that in [5468cc2fb22a0559bf0fd2cfc51ecd17c2b511e7], where I defined the interface (IProperties) of the object passed to getRenderingFor (and, more importantly, for WithProperties callables).

Problem is, the getRenderingFor for _ComputeRepositoryURL expects a build, and calls build.getSourceStamp.

[dustin]

Traceback is something along the lines of

	  File "/Users/derekhurley/Documents/Dropbox/gsoc/bbot/src/master/buildbot/steps/source/oldsource.py", line 37, in getRenderingFor
	    s = build.getSourceStamp()
	exceptions.AttributeError: Properties instance has no attribute 'getSourceStamp'

[dustin]

I fixed this in [9a861eb624264659c05990ed3720fffc4eb361e6] by adding a getBuild method to IProperties.