aggressivemergedelta: document rename and move to `revlog` section

The config does not follow our naming guideline and "Aggressive" is probably a
word to keep away from users.

The option does not truly fit in the `format` section. It can be turned on and
off for existing repository without much consequence regarding compatibility.

A new `revlog` option is created to control behavior related to revlog writing
and reading. We can see multiple other config options that could be migrated
there.

* format.maxchainlen
* experimental.mmapindexthreshold
* experimental.sparse-read.density-threshold (in an updated form)
* experimental.sparse-read.min-gap-size (in an updated form)

In addition, we can foresee at least a couple of sparse-revlog related option
coming too (to reduce delta chain length and increase snapshot reuse)

These two extra options might fit there too. Unless we want to create a
section dedicated to caches and performance.

* format.chunkcachesize
* format.manifestcachesize

For now, we only migrate `optimize-delta-parent-choice` since it is getting
out of experimental. It is too close to the release to move the other one. In
addition, we still lack proper the prioritization of alias that would help
renaming them without bad consequence for users.

(Not fully happy about the `revlog` name but could not find better).

mercurial/configitems.py

@@ -637,9 +637,6 b" coreconfigitem('extdata', '.*',"
     default=None,
     generic=True,
 )
-coreconfigitem('format', 'aggressivemergedeltas',
-    default=True,
-)
 coreconfigitem('format', 'chunkcachesize',
     default=None,
 )
@@ -930,6 +927,10 b" coreconfigitem('progress', 'width',"
 coreconfigitem('push', 'pushvars.server',
     default=False,
 )
+coreconfigitem('revlog', 'optimize-delta-parent-choice',
+    default=True,
+    # formely an experimental option: format.aggressivemergedeltas
+)
 coreconfigitem('server', 'bookmarks-pushkey-compat',
     default=True,
 )

mercurial/help/config.txt

@@ -1773,6 +1773,20 b' have a definite end point.'
 
 Alias definitions for revsets. See :hg:`help revsets` for details.
 
+``revlog``
+----------
+
+Control the strategy Mercurial uses internally to store history. Options in this
+category impact performance and repository size.
+
+``optimize-delta-parent-choice``
+    When storing a merge revision, both parents will be equally considered as
+    a possible delta base. This results in better delta selection and improved
+    revlog compression. This option is enabled by default.
+
+    Turning this option off can result in large increase of repository size for
+    repository with many merges.
+
 ``server``
 ----------
 

mercurial/localrepo.py

@@ -668,9 +668,8 b' class localrepository(object):'
         manifestcachesize = self.ui.configint('format', 'manifestcachesize')
         if manifestcachesize is not None:
             self.svfs.options['manifestcachesize'] = manifestcachesize
-        # experimental config: format.aggressivemergedeltas
-        deltabothparents = self.ui.configbool('format',
-                                              'aggressivemergedeltas')
+        deltabothparents = self.ui.configbool('revlog',
+                                              'optimize-delta-parent-choice')
         self.svfs.options['deltabothparents'] = deltabothparents
         self.svfs.options['lazydeltabase'] = not scmutil.gddeltaconfig(self.ui)
         chainspan = self.ui.configbytes('experimental', 'maxdeltachainspan')

tests/test-generaldelta.t

@@ -118,7 +118,7 b' delta coming from the server base delta '
         2       1        2        0      p1         57        135        161   1.19259       218        57    0.35404
         3       1        2        0      p1         57        135        161   1.19259       275       114    0.70807
 
-Test format.aggressivemergedeltas
+Test revlog.optimize-delta-parent-choice
 
   $ hg init --config format.generaldelta=1 aggressive
   $ cd aggressive
@@ -146,7 +146,7 b' Test format.aggressivemergedeltas'
 - Verify aggressive merge uses p2 (commit 0) as delta parent
   $ hg up -q -C 1
   $ hg merge -q 0
-  $ hg commit -q -m merge --config format.aggressivemergedeltas=True
+  $ hg commit -q -m merge --config revlog.optimize-delta-parent-choice=yes
   $ hg debugdeltachain -m
       rev  chain# chainlen     prev   delta       size    rawsize  chainsize     ratio   lindist extradist extraratio
         0       1        1       -1    base         59        215         59   0.27442        59         0    0.00000