config: pass both relative and absolute paths to `include` callback The `include` callback is responsible for loading configs from `%include` statements. The callback currently gets passed the absolute path [1] to the config to read. That is created by joining the dirname of the file that contains the `%include` statement. For PyOxidizer support, I'm trying to reduce dependence on paths. This patch helps with that by passing the relative path found in the `%include` statement (but with username expansion, etc.) to the `include` callback. It also turns out that the existing callers can easily adapt to using the relative path. Coming patches will clean that up and then we'll remove the absolute path from the callback. [1] The "absolute path" bit is a bit of a lie -- it's going to be an absolute path if the path that was passed into `config.parse()` was absolute. Differential Revision: https://phab.mercurial-scm.org/D8790