fix for FS#2676, inserting zero length spaces into long sequences of non...

...-breaking characters in diffs

post process the html content string returned by Diff->format to locate long, unbroken strings of characters.  Examine those strings and insert zero length (zl) spaces after certain characters (e.g. /#!,:;).  When there are sequences of the 'special' characters only insert the zl space after the last character in the sequence.

Also, don't modify content within html tags and keep html entities together.