Fix year 100 - 999

[MoonE]

This fixes part of #2388

[mgol]

Can you explain precisely what it fixes and what it does not?

[MoonE]

Sorry for the delay, I seem to have missed your comment.

With yy format it expects exactly four digits for the year when parsing.

jquery-ui/ui/widgets/datepicker.js

Lines 1237 to 1240 in e803d4f

	size = ( match === "@" ? 14 : ( match === "!" ? 20 :
	( match === "y" && isDoubled ? 4 : ( match === "o" ? 3 : 2 ) ) ) ),
	minSize = ( match === "y" ? size : 1 ),
	digits = new RegExp( "^\\d{" + minSize + "," + size + "}" ),

Without this change selecting a date with year in range 100-999 works but is formatted as 999-01-01 which cannot be parsed when the datepicker is opened again.

Currently datepicker only works correctly for years 1000-9999.
This makes it work for years 0100-9999,
#2391 would make it work for years 0000-9999