convert: add config option for disabling ancestor parent checks

When converting merge commits, convert checks if any of the parents are
ancestors of any of the other parents. To do this, it builds an ancestor list
for every commit in the repository. On large repos this can take a long time
(30min+). Let's add an option for disabling this check to preserve performance.

The downside of this is that it may create unnecessary parent connections when
enabled (which is unfortunate, but not incorrect).

To verify, I ran the convert tests with the flag enabled, and verified the graph
changes were all just to add new parents that were ancestors of existing
parents.

hgext/convert/filemap.py

@@ -156,6 +156,9 b' class filemap_source(converter_source):'
         self.origparents = {}
         self.children = {}
         self.seenchildren = {}
+        # experimental config: convert.ignoreancestorcheck
+        self.ignoreancestorcheck = self.ui.configbool('convert',
+                                                      'ignoreancestorcheck')
 
     def before(self):
         self.base.before()
@@ -306,7 +309,7 b' class filemap_source(converter_source):'
 
     def getchanges(self, rev, full):
         parents = self.commits[rev].parents
-        if len(parents) > 1:
+        if len(parents) > 1 and not self.ignoreancestorcheck:
             self.rebuild()
 
         # To decide whether we're interested in rev we:
@@ -332,9 +335,11 b' class filemap_source(converter_source):'
             mp1 = self.parentmap[p1]
             if mp1 == SKIPREV or mp1 in knownparents:
                 continue
-            isancestor = any(p2 for p2 in parents
+
+            isancestor = (not self.ignoreancestorcheck and
+                          any(p2 for p2 in parents
                                   if p1 != p2 and mp1 != self.parentmap[p2]
-                                  and mp1 in self.wantedancestors[p2])
+                                 and mp1 in self.wantedancestors[p2]))
             if not isancestor and not hasbranchparent and len(parents) > 1:
                 # This could be expensive, avoid unnecessary calls.
                 if self._cachedcommit(p1).branch == branch: