dirstate-tree: Borrow copy source paths from the "on disk" bytes...
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
