gdatetime: Fix formatting of time zones offsets in range -01:00 to +00:00 (backport to glib-2-58)
Formatting code for
%z specifier incorrectly assumed that sign of
offset from UTC can be recovered from the number of hours alone, which
is not true for offsets between -01:00 and +00:00.
Extract and format sign separately to avoid the problem.
Issue #1337 (closed).
(Backport of !435 (merged) to