userdiff: add builtin driver for INI files

Add a new builtin driver for generic INI files (e. g. the gitconfig files), where: - the funcname regular expression matches section names, i. e. any string between brackets at the beginning of the line, with or without indentation; - word_regex matches any word with one or more non-whitespace characters without checking if it is a valid variable name or value. Also add tests for the new userdiff driver. These files define sections and subsections, with and without indentation. Helped-by: Patrick Steinhardt <ps@pks.im> Helped-by: D. Ben Knoble <ben.knoble@gmail.com> Signed-off-by: Lucas Seiki Oshiro <lucasseikioshiro@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>