There exists a bug in the current `htmldiff` code, where by the generated diff
changes the structure of the html:

```python
>>> from lxml.html import diff
>>> a = "<div id='first'>some old text</div><div id='last'>more old text</div>"
>>> b = "<div id='first'>some old text</div><div id='middle'>and new text</div><div id='last'>more old text</div>"
>>> diff.htmldiff(a, b)
('<div id="middle"> <div id="first"><ins>some old text</ins></div><ins>and new</ins> <del>some old</del> text</div><div id="last">more old text</div>')
>>>

```

This patchset is an attempt to fix that issue.

…s and string interpolation.

…_start

...also some minor clean up.

…merge_insert`

In the recently revised implementation of `merge_insert` we were checking
whether a tag exists in the `balanced/unbalanced` tags list by referring to
just the tag itself. This is unreliable since the same tag might exist in both
list. Incorrect identification leads to skipping of the `<ins>` tags in some
cases, resulting in incorrect diff being rendered.

This pathset fixes the issue described and adds a test to validate it.