Optimize parsing of OSC_STRING to minimize string concatenation.
Optimize parsing of OSC_STRING to minimize string concatenation.

Tyriar/xterm.js,sourcelair/xterm.js,sourcelair/xterm.js,sourcelair/xterm.js,Tyriar/xterm.js,xtermjs/xterm.js,Tyriar/xterm.js,xtermjs/xterm.js,xtermjs/xterm.js,Tyriar/xterm.js,xtermjs/xterm.js,Tyriar/xterm.js,xtermjs/xterm.js,sourcelair/xterm.js