revset: make revsbetween public

This patch is part of a series of patches to speed up the computation of
revset.revsbetween 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 revsbetween is 10-50x faster and smartlog on is
2x-5x faster.

Later in this serie, we want to reuse the implementation of revsbetween in the
changelog module, therefore, we make it public.

mercurial/revset.py

@@ -87,7 +87,7 b' def _revdescendants(repo, revs, followfi'
 
     return generatorset(iterate(), iterasc=True)
 
-def _revsbetween(repo, roots, heads):
+def revsbetween(repo, roots, heads):
     """Return all paths between roots and heads, inclusive of both endpoint
     sets."""
     if not roots:
@@ -406,7 +406,7 b' def rangeset(repo, subset, x, y):'
 
 def dagrange(repo, subset, x, y):
     r = fullreposet(repo)
-    xs = _revsbetween(repo, getset(repo, r, x), getset(repo, r, y))
+    xs = revsbetween(repo, getset(repo, r, x), getset(repo, r, y))
     # XXX We should combine with subset first: 'subset & baseset(...)'. This is
     # necessary to ensure we preserve the order in subset.
     return xs & subset