Handle multiple non-contributing commits being merged

When encountering multiple non-contributing commits included in a merge
that is TREESAME to the existing contents, ensure a list of all commits
being brought in by that merge are included in the list to be pruned
from history.

Change-Id: I643e0923f8967f9b70d10b8796688b782681bfe0
Closes-Bug: #1648029

git_upstream/lib/searchers.py

@@ -132,10 +132,15 @@ class Searcher(GitMixin):
         # happens to be in a list of commits directly between the tip and
         # previous upstream import, then it is simply a merge of an obsolete
         # commit. These are very difficult to strip out normally
-        if any(self.git.merge_base(
-                parent, p, with_exceptions=False) in ancestry_commits
-               for p in other_parents):
-            return None, [], other_parents
+        exclude_commits = []
+        for p in other_parents:
+            if self.git.merge_base(
+                    p, parent, with_exceptions=False) in ancestry_commits:
+                exclude_commits.extend(
+                    self.git.rev_list("%s..%s" % (parent, p)).splitlines())
+
+        if exclude_commits:
+            return None, [], exclude_commits
 
         # otherwise looking at the previous import merge commit and the parent
         # from the previous import branch, so exclude all other parents.

git_upstream/tests/searchers/scenarios/treesame_change_approved_after_import_merge.yaml

@@ -12,7 +12,7 @@
 
     Repository layout being tested
 
-              O------------
+              O---P--------
              /             \
         B---C---D---G---H---I---J---K   master
        /               /
@@ -26,6 +26,7 @@
     - [B, [A]]
     - [C, [B]]
     - [O, [C]]
+    - [P, [O]]
     - [D, [C]]
     - [E, [A]]
     - [F, [E]]
@@ -35,7 +36,7 @@
     - [D1, [C1]]
     - [H, [G, =D1]]
     # next node definition ensures I is TREESAME to H
-    - [I, [=H, O]]
+    - [I, [=H, P]]
     - [J, [I]]
     - [K, [J]]
     - [L, [F]]

git_upstream/tests/searchers/scenarios/treesame_change_approved_no_previous_import.yaml

@@ -12,7 +12,7 @@
 
     Repository layout being tested
 
-            O----
+            O---P
            /     \
       B---C---D---G---H                 master
      /
@@ -24,10 +24,11 @@
     - [B, [A]]
     - [C, [B]]
     - [O, [C]]
+    - [P, [O]]
     - [D, [C]]
     - [E, [A]]
     - [F, [E]]
-    - [G, [=D, O]]
+    - [G, [=D, P]]
     - [H, [G]]
     - [L, [F]]
     - [M, [L]]