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>