doc: pretty-formats note wide char limitations, and add tests

The previous commits added clarifications to the column alignment placeholders, note that the spaces are optional around the parameters. Also, a proposed extension [1] to allow hard truncation (without ellipsis '..') highlighted that the existing code does not play well with wide characters, such as Asian fonts and emojis. For example, N wide characters take 2N columns so won't fit an odd number column width, causing misalignment somewhere. Further analysis also showed that decomposed characters, e.g. separate `a` + `umlaut` Unicode code-points may also be mis-counted, in some cases leaving multiple loose `umlauts` all combined together. Add some notes about these limitations, and add basic tests to demonstrate them. The chosen solution for the tests is to substitute any wide character that overlaps a splitting boundary for the unicode vertical ellipsis code point as a rare but 'obvious' substitution. An alternative could be the substitution with a single dot '.' which matches regular expression usage, and our two dot ellipsis, and further in scenarios where the bulk of the text is wide characters, would be obvious. In mainly 'ascii' scenarios a singleton emoji being substituted by a dot could be confusing. It is enough that the tests fail cleanly. The final choice for the substitute character can be deferred. [1] https://lore.kernel.org/git/20221030185614.3842-1-philipoakley@iee.email/ Signed-off-by: Philip Oakley <philipoakley@iee.email> Signed-off-by: Junio C Hamano <gitster@pobox.com>