Use the Rectangular Marquee Tool and select which part you would like the 'stretch' to start. Be sure that the width is the smallest possible selection because stretching a selection with a wider width often produces an 'uneven colored stretch.
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Copy (CTRL+C) and paste (CTRL+V). A new layer surfaceth.
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Now Edit » Transform » Scale and drag the right part to do the 'stretch'. And there you have it. Feel free to use your creativity and some other processes to come up a version of your stretch like the ones in the image below.
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