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
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