changelog: add way to call the reachableroots C implementation

This patch is part of a series of patches to speed up the computation of
revset.reachableroots by introducing a C implementation. The main motivation is
to speed up smartlog on big repositories. At the end of the series, on our big
repositories the computation of reachableroots is 10-50x faster and smartlog on
is 2x-5x faster.

This patch allows us to call the new C implementation of reachableroots from
python by creating an entry point in the changelog class.

mercurial/changelog.py

@@ -18,6 +18,7 b' from . import ('
     encoding,
     error,
     revlog,
+    revset,
     util,
 )
 
@@ -184,6 +185,16 b' class changelog(revlog.revlog):'
         self.rev(self.node(0))
         return self._nodecache
 
+    def reachableroots(self, minroot, heads, roots, includepath=False):
+        reachable = self.index.reachableroots(minroot, heads, roots,
+                                              includepath)
+        if reachable is None:
+            # The C code hasn't been able to initialize a list, something went
+            # really wrong, let's rely on the pure implementation in that case
+            raise AttributeError()
+        else:
+            return revset.baseset(sorted(reachable))
+
     def headrevs(self):
         if self.filteredrevs:
             try: