revlog: accept a revs argument in `headrevs`

Computing the heads of an arbitrary set of revision is useful, we make it
possible to do so through the `headrevs` method of the revlog.

Right now, this is just calling dagop's implementation. However, we expect to
plug a native implementation soon.

mercurial/changelog.py

@@ -347,8 +347,8 b' class changelog(revlog.revlog):'
     def reachableroots(self, minroot, heads, roots, includepath=False):
         return self.index.reachableroots2(minroot, heads, roots, includepath)
 
-    def headrevs(self):
-        if self.filteredrevs:
+    def headrevs(self, revs=None):
+        if revs is None and self.filteredrevs:
             try:
                 return self.index.headrevsfiltered(self.filteredrevs)
             # AttributeError covers non-c-extension environments and
@@ -356,7 +356,7 b' class changelog(revlog.revlog):'
             except AttributeError:
                 return self._headrevs()
 
-        return super(changelog, self).headrevs()
+        return super(changelog, self).headrevs(revs)
 
     def strip(self, *args, **kwargs):
         # XXX make something better than assert

mercurial/revlog.py

@@ -1102,11 +1102,13 b' class revlog(object):'
         assert heads
         return (orderedout, roots, heads)
 
-    def headrevs(self):
+    def headrevs(self, revs=None):
+        if revs is None:
         try:
             return self.index.headrevs()
         except AttributeError:
             return self._headrevs()
+        return dagop.headrevs(revs, self.parentrevs)
 
     def computephases(self, roots):
         return self.index.computephasesmapsets(roots)