t3701: add a test for advanced split-hunk editing

t3701: add a test for advanced split-hunk editing

In this developer's workflows, it often happens that a hunk needs to be
edited in a way that adds lines, and sometimes even reduces the number
of context lines.

Let's add a regression test for this.

Note that just like the preceding test case, the new test case is *not*
handled gracefully by the current `git add -p`. It will be handled
correctly by the upcoming built-in `git add -p`, though.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>