Make negative time zones produce '-700', not '--700'
Make negative time zones produce '-700', not '--700'

This fixes failing spec:
'Gitrb::Commit should be readable by git binary' FAILED
expected: "Date:   Mon Apr 20 00:00:00 2009 -0700\n",
     got: "Date:   Mon Apr 20 00:00:00 2009 +0000\n" (using ==)
     commit_spec.rb:45:in `block (3 levels) in <top (required)>'

minad/gitrb