ui: add a parameter to set the temporary directory for edit Until callsites are updated, this will have no effect. Once callsites are updated, specifying experimental.editortmpinhg will create editor temporary files in a subdirectory of .hg, which will make it easier for tool integrations to determine what repository is in play when they're asked to edit an hg-related file.

help: update help for `hg update` which was misleading (issue5427)

templater: add '{envvars}' to access environment variables Since the option for ui.exportableenviron is experimental, so is this template until the underlying API is sorted out.

ui: introduce an experimental dict of exportable environment variables Care needs to be taken to prevent leaking potentially sensitive environment variables through hgweb, if template support for environment variables is to be introduced. There are a few ideas about the API for preventing accidental leaking [1]. Option 3 seems best from the POV of not needing to configure anything in the normal case. I couldn't figure out how to do that, so guard it with an experimental option for now. [1] https://www.mercurial-scm.org/pipermail/mercurial-devel/2017-January/092383.html

tests: test experimental.spacemovesdown config for commit -i The feature is still very experimental, but at least its behavior is captured in the test.

zstd: prevent potential free() of uninitialized memory This is a cherry pick of an upstream fix. The free() of uninitialed memory could likely only occur if a malloc() inside zstd fails. The patched functions aren't currently used by Mercurial. But I don't like leaving footguns sitting around.