context: some gratuitous documentation improvement I wrote it as I was reading the code.

profiling: add a py-spy profiling backend The recommended way to use this backend is by setting the config `profiling.output` to point to a file because py-spy output is not human-readable.

copytracing: fix a bug in an edge case in metadata.compute_all_files_changes

rhg: ignore readonly FS error when saving dirstate The error is already ignored when the .hg directory is read-only, so this is only fair. (the python hg is silent on readonly fs, too)