windows: don't consider '$$' to be an escaped '$' when translating to cmd.exe This functionality was inherited from `os.path.expandvars()`. But the point of adding this translating code is to be able to write a portable hook, and bash wouldn't replace '$$' with '$'. Escaping with '\' works, and is portable.

contrib: add heads(commonancestors(_)) to all-revsets This is mainly to check that we don't regress our optimization path.