[kritagyadubey]

When a summary has more than one sentence and is followed by a description,
do_split_summary splits at the first period but then places the rest of the
summary in the wrong position. On master this mangles the docstring: a blank line
ends up between the first and second sentence, and the description is appended to
the end of the summary with extra spaces.

Given:

def foo():
    """First sentence is here. Second sentence is long and split in two. I even have a third sentence here.

    And other text here.
    """

master produces:

def foo():
    """First sentence is here.

    Second sentence is long and split in two. I even have a third sentence
    here.     And other text here.
    """

The rest of a multi-sentence summary still belongs to the summary, so this inserts
it directly after the first sentence and leaves the blank line where it separates
the summary from the description:

def foo():
    """First sentence is here. Second sentence is long and split in two. I even
    have a third sentence here.

    And other text here.
    """

This adds unit tests for do_split_summary (single-line and two-line summaries)
and an end-to-end case in do_format_code.toml. Both fail on master and pass
with the change.

The original report in #315 showed the summary lines reordered. That exact
reordering no longer occurs after later refactors, but the summary/description
boundary is still mangled on master, so the issue remains open.

Fixes #315