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.