[PATCH 1 of 5 V2] config: add option to control creation of empty successors during rewrite

[Manuel Jacob]

# HG changeset patch
# User Manuel Jacob <me at manueljacob.de>
# Date 1594504407 -7200
#      Sat Jul 11 23:53:27 2020 +0200
# Node ID 8263ac153567482d8794d76a897eb47a0f116e2d
# Parent  52fad88f31b2a3dc16ea8654147bb6ff31482d26
# EXP-Topic keep_empty_successor
config: add option to control creation of empty successors during rewrite

The default for many history-rewriting commands (e.g. rebase and absorb) is
that changesets which would become empty are not created in the target branch.
This makes sense if the source branch consists of small fix-up changes. For
more advanced workflows that make heavy use of history-editing to create
curated patch series, dropping empty changesets is not as important or even
undesirable.

Some users want to keep the meta-history, e.g. to make finding comments in a
code review tool easier or to avoid that divergent bookmarks are created. For
that, obsmarkers from the (to-be) empty changeset to the changeset(s) that
already made the changes should be added. If a to-be empty changeset is pruned
without a successor, adding the obsmarkers is hard because the changeset has to
be found within the hidden part of the history.

If rebasing in TortoiseHg, it’s easy to miss the fact that the to-be empty
changeset was pruned. An empty changeset will function as a reminder that
obsmarkers should be added.

Martin von Zweigbergk mentioned another advantage. Stripping the successor will
de-obsolete the predecessor. If no (empty) successor is created, this won’t be
possible.

In the future, we may want to consider other behaviors, like e.g. creating the
empty successor, but pruning it right away. Therefore this configuration
accepts 'skip' and 'keep' instead of being a boolean configuration.

diff --git a/mercurial/configitems.py b/mercurial/configitems.py
--- a/mercurial/configitems.py
+++ b/mercurial/configitems.py
@@ -1068,6 +1068,9 @@
     b'rewrite', b'update-timestamp', default=False,
 )
 coreconfigitem(
+    b'rewrite', b'empty-successor', default=b'skip', experimental=True,
+)
+coreconfigitem(
     b'storage', b'new-repo-backend', default=b'revlogv1', experimental=True,
 )
 coreconfigitem(
diff --git a/mercurial/helptext/config.txt b/mercurial/helptext/config.txt
--- a/mercurial/helptext/config.txt
+++ b/mercurial/helptext/config.txt
@@ -1890,6 +1890,14 @@
     applicable for `hg amend`, `hg commit --amend` and `hg uncommit` in the
     current version.
 
+``empty-successor``
+
+    Control what happens with empty successors that are the result of rewrite
+    operations. If set to ``skip``, the successor is not created. If set to
+    ``keep``, the empty successor is created and kept.
+
+    Currently, no command considers this configuration. (EXPERIMENTAL)
+
 ``storage``
 -----------