Add test demonstrating problem with staging only some changed lines

Given a block of consecutive changed lines, trying to stage only some of them
doesn't work correctly in all cases:

- if the staged lines are the last lines in the block of changes, it already
  works
- when staging some changes in the middle of the block, it doesn't work as
  desired, but we also don't try to fix this case in this branch, because it's
  harder to do, and not as common as the other two
- staging the first lines of the block doesn't work as desired, and we will fix
  that in this branch.

pkg/integration/tests/test_list.go

@@ -379,6 +379,9 @@ var tests = []*components.IntegrationTest{
 	staging.Search,
 	staging.StageHunks,
 	staging.StageLines,
+	staging.StagePartialBlockOfChangesFirstLines,
+	staging.StagePartialBlockOfChangesLastLines,
+	staging.StagePartialBlockOfChangesMiddleLines,
 	staging.StageRanges,
 	stash.Apply,
 	stash.ApplyPatch,