Make "Find base commit for fixup" work with hunks with only added lines

To understand what this does and why, read the design document that I'm about to
add in the next commit.

pkg/integration/tests/test_list.go

@@ -79,6 +79,7 @@ var tests = []*components.IntegrationTest{
 	commit.CreateTag,
 	commit.DiscardOldFileChanges,
 	commit.FindBaseCommitForFixup,
+	commit.FindBaseCommitForFixupOnlyAddedLines,
 	commit.FindBaseCommitForFixupWarningForAddedLines,
 	commit.Highlight,
 	commit.History,