Ported from FB 470019.

epriestley> We should probably fix this at some point but nearly no one is impacted and the damage is very small.

There is an issue with, roughly, "svn rm x; svn commit; svn cp x@12345 y; touch x; svn add x; arc diff", creating a diff where a path was both a copy origin (at a different rev) and a new file.

I pushed this to fix the immediate issue (arc failing with an exception):

https://github.com/facebook/arcanist/commit/f071a552667f2d1d58582ff63c3f4e6c70e808ea

Differential does not display the diff completely correctly. The files should say "this file was not changed _click here to view contents_", not "This file was changed only by adding or removing trailing whitespace.". The data is present in the database so this is just a display problem.

Putting it at low-pri since it shouldn't really block anything too badly (reviewers can go look in Diffusion for file content; inelegant but doable) and this will be easiest to fix when I bring Differential over to open source.