dirstate-tree: Borrow copy source paths from the "on disk" bytes Use std::borrow::Cow to avoid some memory allocations and copying. These particular allocations are not visible when profiling (as many files in a typical repo don’t have a copy source). This change is "warm up" for doing the same with paths of files themselves, which is more involved since those paths are used as `HashMap` keys. This gets of the way the addition of a lifetime parameter to several types. Differential Revision: https://phab.mercurial-scm.org/D10559