context: add specialized way of getting copy source file only I'm working on support for storing copy metadata in the changeset instead of the filelog. I don't intend to include the file nodeid there, but most callers don't need that anyway. This patch introduces a method similar to ctx.renamed(), but the new method returns only the source filename, not the nodeid. Subsequent patches will move callers over to this new method. Differential Revision: https://phab.mercurial-scm.org/D6008