templater: switch to lower-level config.parse() in _readmapfile() I hope to modify this code to also work with resources loaded from memory (for PyOxidizer support). For that, we'll need to handle the lookup of relative `%include` path (not joined with the base directory of the containing file). This patch prepares for that by using `config.parse()` instead of `config.read()`, since the latter expects a file in the file system. As it happens, this change also lets us clean up the `config` class to not need the `_includepaths` field. That will happen next. Differential Revision: https://phab.mercurial-scm.org/D8791

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

tests: add test for bad template %include and __base__ It doesn't seem like we had any tests for `%include non-existent` and `__base__ = non-existent`. The latter raises an error while the former ignores the include. We should probably also make the former an error. Differential Revision: https://phab.mercurial-scm.org/D8798

tests: add test for unqualified include of a built-in template map The feature was added in 081b08e4ea13 (templater: look for mapfiles in template paths, 2015-05-15), but no test case was added. Differential Revision: https://phab.mercurial-scm.org/D8797