gitweb: Fix parsing of negative fractional timezones in JavaScript

gitweb: Fix parsing of negative fractional timezones in JavaScript

Extract converting numerical timezone in the form of '(+|-)HHMM' to
timezoneOffset function, and fix parsing of negative fractional
timezones.

This is used to format timestamps in 'blame_incremental' view; this
complements commit 2b1e172 (gitweb: Fix handling of fractional
timezones in parse_date, 2011-03-25).

Now

  gitweb.cgi/git.git/blame_incremental/3fe5489:/contrib/gitview/gitview#l853

and

  gitweb.cgi/git.git/blame/3fe5489:/contrib/gitview/gitview#l853

show the same correct time in author's local timezone in title
(on mouseover) [Aneesh Kumar K.V, 2006-02-24 00:59:42 +0530].

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>