to_char(): Do not count negative sign as a digit for time values

to_char(): Do not count negative sign as a digit for time values

For time masks, like HH24, MI, SS, CC, MM, do not count the negative
sign as part of the zero-padding length specified by the mask, e.g. have
to_char('-4 years'::interval, 'YY') return '-04', not '-4'.

Report by Craig Ringer