unicode_width.h: rename to use dash in file name

This is more consistent with the project style. The majority of Git's
source files use dashes in preference to underscores in their file names.

Also adjust contrib/update-unicode as well.

Signed-off-by: Stefan Beller <[e-mail]>

contrib/update-unicode/README

···
 TL;DR: Run update_unicode.sh after the publication of a new Unicode
-standard and commit the resulting unicode_widths.h file.
+standard and commit the resulting unicode-widths.h file.
 
 The long version
 ================
 
-The Git source code ships the file unicode_widths.h which contains
+The Git source code ships the file unicode-widths.h which contains
 tables of zero and double width Unicode code points, respectively.
 These tables are generated using update_unicode.sh in this directory.
 update_unicode.sh itself uses a third-party tool, uniset, to query two
···
 
 On each run, update_unicode.sh checks whether more recent Unicode data
 files are available from the Unicode consortium, and rebuilds the header
-unicode_widths.h with the new data. The new header can then be
+unicode-widths.h with the new data. The new header can then be
 committed.

contrib/update-unicode/update_unicode.sh

···
 #Cf Format          a format control character
 #
 cd "$(dirname "$0")"
-UNICODEWIDTH_H=$(git rev-parse --show-toplevel)/unicode_width.h
+UNICODEWIDTH_H=$(git rev-parse --show-toplevel)/unicode-width.h
 
 wget -N http://www.unicode.org/Public/UCD/latest/ucd/UnicodeData.txt \
 	http://www.unicode.org/Public/UCD/latest/ucd/EastAsianWidth.txt &&

utf8.c

···
 	/*
 	 * Sorted list of non-overlapping intervals of non-spacing characters,
 	 */
-#include "unicode_width.h"
+#include "unicode-width.h"
 
 	/* test for 8-bit control characters */
 	if (ch == 0)