utf8: make it easier to auto-update git_wcwidth()

The function git_wcwidth() returns for a given unicode code point the
width on the display:

 -1 for control characters,
  0 for combining or other non-visible code points
  1 for e.g. ASCII
  2 for double-width code points.

This table had been originally been extracted for one Unicode
version, probably 3.2.

We now use two tables these days, one for zero-width and another for
double-width.  Make it easier to update these tables to a later
version of Unicode by factoring out the table from utf8.c into
unicode_width.h and add the script update_unicode.sh to update the
table based on the latest Unicode specification files.

Thanks to Peter Krefting <peter@softwolves.pp.se> and Kevin Bracey
<kevin@bracey.fi> for helping with their Unicode knowledge.

Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

update_unicode.sh

@@ -0,0 +1,37 @@
+#!/bin/sh
+#See http://www.unicode.org/reports/tr44/
+#
+#Me Enclosing_Mark  an enclosing combining mark
+#Mn Nonspacing_Mark a nonspacing combining mark (zero advance width)
+#Cf Format          a format control character
+#
+UNICODEWIDTH_H=../unicode_width.h
+if ! test -d unicode; then
+	mkdir unicode
+fi &&
+( cd unicode &&
+	if ! test -f UnicodeData.txt; then
+		wget http://www.unicode.org/Public/UCD/latest/ucd/UnicodeData.txt
+	fi &&
+	if ! test -f EastAsianWidth.txt; then
+		wget http://www.unicode.org/Public/UCD/latest/ucd/EastAsianWidth.txt
+	fi &&
+	if ! test -d uniset; then
+		git clone https://github.com/depp/uniset.git
+	fi &&
+	(
+		cd uniset &&
+		if ! test -x uniset; then
+			autoreconf -i &&
+			./configure --enable-warnings=-Werror CFLAGS='-O0 -ggdb'
+		fi &&
+		make
+	) &&
+	echo "static const struct interval zero_width[] = {" >$UNICODEWIDTH_H &&
+	UNICODE_DIR=. ./uniset/uniset --32 cat:Me,Mn,Cf + U+1160..U+11FF - U+00AD |
+	grep -v plane >>$UNICODEWIDTH_H &&
+	echo "};" >>$UNICODEWIDTH_H &&
+	echo "static const struct interval double_width[] = {" >>$UNICODEWIDTH_H &&
+	UNICODE_DIR=. ./uniset/uniset --32 eaw:F,W >>$UNICODEWIDTH_H &&
+	echo "};" >>$UNICODEWIDTH_H
+)