hooks: introduce a `:run-with-plain` option for hooks This option control if HGPLAIN should be set or not for the hooks. This is the first step to give user some control of the HGPLAIN setting for they hooks. Some hooks (eg: consistency checking) deserve to be run with HGPLAIN, some other (eg: user set visual helper) might need to respect the user config and setting. So both usage are valid and we need to restore the ability to run -without- HGPLAIN that got lost in Mercurial 5.7. This does not offer a way to restore the pre-5.7 behavior yet (respect whatever HGPLAIN setting from the shell), this will be dealt with in the next changeset. The option name is a bit verbose because implementing this highlighs the need for another option: `:run-if-plain`. That would make it possible for some hooks to be easily disabled if HG PLAIN is set. However such option would be a new feature, not something introduced to mitigate a behavior change introduced in 5.7, so the `:run-if-plain` option belong to the default branch and is not part of this series. Differential Revision: https://phab.mercurial-scm.org/D9981

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