annotate .clang-format @ 47125:9be618452c3b

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

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