rebase: add boolean config item rebase.store-source

rebase: add boolean config item rebase.store-source

This allows to use rebase without recording a rebase_source extra
field. This is useful for example to build a mirror converted from
another SCM (such as svn) by converting only new revisions, and
then incrementally add them to the destination by pulling from the
newly converted (unrelated) repo and rebasing the new revisions
onto the last old already stored changeset. Without this patch the
rebased changesets would always receive some rebase_source that
would depend on the particular history of the conversion process,
instead of only depending on the original source revisions.

This is used to implement a hg mirror repo of SvarDOS (a partially
nonfree but completely redistributable DOS distribution) in the
scripts at https://hg.pushbx.org/ecm/svardos.scr/

In particular, cre.sh creates an svn mirror, upd.sh recreates an
entire hg repo from the svn mirror (which takes too long to do in a
regular job), and akt.sh uses hg convert with the config item
convert.svn.startrev to incrementally convert only the two most
recent revisions already found in the mirror destination plus any
possible new revisions. If any are found, the temporary repo's
changesets are pulled into the destination (as changesets from an
unrelated repository). Then the changesets corresponding to the new
revisions are rebased onto the prior final changeset. (Finally, the
two remaining duplicates of the prior head and its parent are
stripped from the destination repository.)

Without this patch, the particular rebase_source extra field would
depend on the order and times at which akt.sh was used, instead of
only depending on the source repository. In other words, whatever
sequence of upd.sh and akt.sh is used at whatever times, it is
desired that the final output repositories always match each other
exactly.

--- a/hgext/rebase.py	Mon Jul 25 05:30:06 2022 +0200
+++ b/hgext/rebase.py	Wed Apr 20 19:24:39 2022 +0200
@@ -546,7 +546,9 @@
         date = self.date
         if date is None:
             date = ctx.date()
-        extra = {b'rebase_source': ctx.hex()}
+        extra = {}
+        if repo.ui.configbool(b'rebase', b'store-source'):
+            extra = {b'rebase_source': ctx.hex()}
         for c in self.extrafns:
             c(ctx, extra)
         destphase = max(ctx.phase(), phases.draft)

--- a/mercurial/configitems.py	Mon Jul 25 05:30:06 2022 +0200
+++ b/mercurial/configitems.py	Wed Apr 20 19:24:39 2022 +0200
@@ -2841,3 +2841,17 @@
     b'experimental.inmemory',
     default=False,
 )
+
+# This setting controls creation of a rebase_source extra field
+# during rebase. When False, no such field is created. This is
+# useful eg for incrementally converting changesets and then
+# rebasing them onto an existing repo.
+# WARNING: this is an advanced setting reserved for people who know
+# exactly what they are doing. Misuse of this setting can easily
+# result in obsmarker cycles and a vivid headache.
+coreconfigitem(
+    b'rebase',
+    b'store-source',
+    default=True,
+    experimental=True,
+)