utf8.c: use a table for double_width

Refactor git_wcwidth() and replace the if-else-if chain.
Use the table double_width which is scanned by the bisearch() function,
which is already used to find combining code points.

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