bisect-kit: Revert cache lookup for is_ancestor_commit

This cache logic is incorrect after two commits merged and being cached
in a same branch cache list. In this situation, there is no way to
identify if two commits are merged and causes incorrect result.

BUG=b:159418757
TEST=./bisect_cros_repo.py init --old R85-13279.0.0 --new R85-13280.0.0

Change-Id: Ieda523c9c1cd19ee17287a8c20c9002517b305ef
diff --git a/bisect_kit/git_util.py b/bisect_kit/git_util.py
index fb60a26..2994fd3 100644
--- a/bisect_kit/git_util.py
+++ b/bisect_kit/git_util.py
@@ -260,13 +260,6 @@
       return True
     raise FastLookupFailed
 
-  def is_ancestor_commit(self, old, new):
-    old_idx = self.commit_to_index.get(old)
-    new_idx = self.commit_to_index.get(new)
-    if old_idx is not None and new_idx is not None:
-      return old_idx < new_idx
-    raise FastLookupFailed
-
 
 class FastLookup:
   """Collection of FastLookupEntry"""
@@ -308,18 +301,6 @@
         pass
     raise FastLookupFailed
 
-  def is_ancestor_commit(self, git_repo, old, new):
-    # This function is optimized only after get_rev_by_time() is invoked.
-    if git_repo not in self.entries:
-      raise FastLookupFailed
-
-    for entry in self.entries[git_repo].values():
-      try:
-        return entry.is_ancestor_commit(old, new)
-      except FastLookupFailed:
-        pass
-    raise FastLookupFailed
-
 
 fast_lookup = FastLookup()
 
@@ -350,6 +331,7 @@
     return False
 
 
+@cache_util.Cache.default_disabled
 def is_ancestor_commit(git_repo, old, new):
   """Determines `old` commit is ancestor of `new` commit.
 
@@ -362,11 +344,6 @@
     True only if `old` is the ancestor of `new`. One commit is not considered
     as ancestor of itself.
   """
-  try:
-    return fast_lookup.is_ancestor_commit(git_repo, old, new)
-  except FastLookupFailed:
-    pass
-
   return util.check_output(
       'git',
       'rev-list',