relnotes: elaborate on the side effects of hooks having HGPLAIN=1

Differential Revision: https://phab.mercurial-scm.org/D9943

relnotes/next

@@ -1,66 +1,70
 == New Features ==
 
  * There is a new config section for templates used by hg commands. It
    is called `[command-templates]`. Some existing config options have
    been deprecated in favor of config options in the new
    section. These are: `ui.logtemplate` to `command-templates.log`,
    `ui.graphnodetemplate` to `command-templates.graphnode`,
    `ui.mergemarkertemplate` to `command-templates.mergemarker`,
    `ui.pre-merge-tool-output-template` to
    `command-templates.pre-merge-tool-output`.
 
  * There is a new set of config options for the template used for the
    one-line commit summary displayed by various commands, such as `hg
    rebase`. The main one is `command-templates.oneline-summary`. That
    can be overridden per command with
    `command-templates.oneline-summary.<command>`, where `<command>`
    can be e.g. `rebase`. As part of this effort, the default format
    from `hg rebase` was reorganized a bit.
 
  * `hg strip`, from the strip extension, is now a core command, `hg
    debugstrip`. The extension remains for compatibility.
 
  * `hg diff` and `hg extdiff` now support `--from <rev>` and `--to <rev>`
    arguments as clearer alternatives to `-r <revs>`. `-r <revs>` has been
    deprecated.
 
  * The memory footprint per changeset during pull/unbundle
    operations has been further reduced.
 
  * There is a new internal merge tool called `internal:mergediff` (can
    be set as the value for the `merge` config in the `[ui]`
    section). It resolves merges the same was as `internal:merge` and
    `internal:merge3`, but it shows conflicts differently. Instead of
    showing 2 or 3 snapshots of the conflicting pieces of code, it
    shows one snapshot and a diff. This may be useful when at least one
    side of the conflict is similar to the base. The new marker style
    is also supported by "premerge" as
    `merge-tools.<tool>.premerge=keep-mergediff`.
 
- * External hooks are now called with `HGPLAIN=1` preset.
+ * External hooks are now called with `HGPLAIN=1` preset.  This has the side
+   effect of ignoring aliases, templates, revsetaliases, and a few other config
+   options in any `hg` command spawned by the hook.  The previous behavior
+   can be restored by setting HGPLAINEXCEPT appropriately in the parent process.
+   See `hg help environment` for the list of items, and how to set it.
 
  * The `branchmap` cache is updated more intelligently and can be
    significantly faster for repositories with many branches and changesets.
 
 
 == New Experimental Features ==
 
 * `experimental.single-head-per-branch:public-changes-only` can be used
   restrict the single head check to public revision. This is useful for
   overlay repository that have both a publishing and non-publishing view
   of the same storage.
 
 
 == Bug Fixes ==
 
 
 
 == Backwards Compatibility Changes ==
 
  * `--force-lock` and `--force-wlock` options on `hg debuglock` command are
    renamed to `--force-free-lock` and `--force-free-wlock` respectively.
 
 
 == Internal API Changes ==