rhg: Switch rhg.ignored-extensions config to Python-compatible list syntax This includes the ability to have an extension name/path that contains a comma, by double-quoting it. Differential Revision: https://phab.mercurial-scm.org/D11390

rhg: Port Python’s `ui.configlist` as `Config::get_list` This new method is not used yet outside of its own unit tests, so this changeset should make no observable change. The Rust parser implementation attempts to exactly replicate the behavior of the Python one, even in edge cases where that behavior is… surprising. New unit tests capture some of these edge cases. This started as a line-by-line port. The main changes are: * Pass around a parser mode enum instead of parser functions * Inline the whole parser into one function * Use `[u8]::get` which returns an `Option`, instead of indexing after explicitly checking the length. Differential Revision: https://phab.mercurial-scm.org/D11389

rust: Generalize the `trim_end_newlines` utility of byte strings … into `trim_end_matches` that takes a callack. Also add `trim_start_matches`. Differential Revision: https://phab.mercurial-scm.org/D11388