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/codechange.py b/bisect_kit/codechange.py
index afde5ed..3aadf79 100644
--- a/bisect_kit/codechange.py
+++ b/bisect_kit/codechange.py
@@ -923,6 +923,7 @@
     git_util.get_commit_metadata.enable_cache()
     git_util.get_file_from_revision.enable_cache()
     git_util.is_containing_commit.enable_cache()
+    git_util.is_ancestor_commit.enable_cache()
 
     # step 1, find all float and fixed specs in the given range.
     fixed_specs = self.spec_manager.collect_fixed_spec(old, new)
@@ -1018,6 +1019,7 @@
     git_util.get_commit_metadata.disable_cache()
     git_util.get_file_from_revision.disable_cache()
     git_util.is_containing_commit.disable_cache()
+    git_util.is_ancestor_commit.disable_cache()
 
     return revlist, details