changelog-v2: add a configuration to disable rank computation

We encountered a graph where rank computation was pathologically slow. We add an
option to disable this computation while this is getting fixed.

Disabling the rank computation should allow for testing other changelog-v2
features undisturbed (like changeset-based copy tracing).

I am purposely not adding a test for the new non-default code path, as this is
a temporary work around of an experimental feature.

mercurial/configitems.py

@@ -923,6 +923,13 b' coreconfigitem('
     b'changegroup4',
     default=False,
 )
+
+# might remove rank configuration once the computation has no impact
+coreconfigitem(
+    b'experimental',
+    b'changelog-v2.compute-rank',
+    default=True,
+)
 coreconfigitem(
     b'experimental',
     b'cleanup-as-archived',

mercurial/localrepo.py

@@ -1068,6 +1068,8 b' def resolverevlogstorevfsoptions(ui, req'
         options[b'revlogv2'] = True
     if requirementsmod.CHANGELOGV2_REQUIREMENT in requirements:
         options[b'changelogv2'] = True
+        cmp_rank = ui.configbool(b'experimental', b'changelog-v2.compute-rank')
+        options[b'changelogv2.compute-rank'] = cmp_rank
 
     if requirementsmod.GENERALDELTA_REQUIREMENT in requirements:
         options[b'generaldelta'] = True

mercurial/revlog.py

@@ -365,6 +365,11 b' class revlog:'
         self._srdensitythreshold = 0.50
         self._srmingapsize = 262144
 
+        # other optionnals features
+
+        # might remove rank configuration once the computation has no impact
+        self._compute_rank = False
+
         # Make copy of flag processors so each revlog instance can support
         # custom flags.
         self._flagprocessors = dict(flagutil.flagprocessors)
@@ -406,6 +411,7 b' class revlog:'
 
         if b'changelogv2' in opts and self.revlog_kind == KIND_CHANGELOG:
             new_header = CHANGELOGV2
+            self._compute_rank = opts.get(b'changelogv2.compute-rank', True)
         elif b'revlogv2' in opts:
             new_header = REVLOGV2
         elif b'revlogv1' in opts:
@@ -2499,7 +2505,7 b' class revlog:'
             sidedata_offset = 0
 
         rank = RANK_UNKNOWN
-        if self._format_version == CHANGELOGV2:
+        if self._compute_rank:
             if (p1r, p2r) == (nullrev, nullrev):
                 rank = 1
             elif p1r != nullrev and p2r == nullrev: