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>

userdiff.c

@@ -211,6 +211,10 @@ PATTERNS("html",
 	 "^[ \t]*(<[Hh][1-6]([ \t].*)?>.*)$",
 	 /* -- */
 	 "[^<>= \t]+"),
+PATTERNS("ini",
+	 "^[ \t]*\\[[^]]+\\]",
+	 /* -- */
+	 "[^ \t]+"),
 PATTERNS("java",
 	 "!^[ \t]*(catch|do|for|if|instanceof|new|return|switch|throw|while)\n"
 	 /* Class, enum, interface, and record declarations */