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