histedit: add tmpdir parameter to ui.edit call

cmdutil: add tmpdir parament to ui.edit calls

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.

revlog: give EXTSTORED flag value to narrowhg Narrowhg has been using "1 << 14" as its revlog flag value for a long time. We (Google) have many repos with that value in production already. When the same value was reserved for EXTSTORED, it made those repos invalid. Upgrading them will be a little painful. We should clearly have reserved the value for narrowhg a long time ago. Since the EXTSTORED flag is not yet in any release and Facebook also says they have not started using it in production, so it should be okay to change it. This patch gives the current value (1 << 14) back to narrowhg and gives a new value (1 << 13) to EXTSTORED.

help: don't let tools reflow revlog flags list Before this change, the text about revlog flags was reflowed into a single paragraph, which made it a bit hard to read. I don't even know the rules around this, but adding a blank line before each flag seems to prevent the reflowing.