range-diff: add tests

range-diff: add tests

These are essentially lifted from https://github.com/trast/tbdiff, with
light touch-ups to account for the command now being named `git
range-diff`.

Apart from renaming `tbdiff` to `range-diff`, only one test case needed
to be adjusted: 11 - 'changed message'.

The underlying reason it had to be adjusted is that diff generation is
sometimes ambiguous. In this case, a comment line and an empty line are
added, but it is ambiguous whether they were added after the existing
empty line, or whether an empty line and the comment line are added
*before* the existing empty line. And apparently xdiff picks a different
option here than Python's difflib.

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