This line is an example of a metaphor.
You asked whether this phrase was an example of a metaphor or personification. A metaphor is a direct comparison of two unlike things, whereas personification is describing something nonhuman by giving it human qualities.
In this case, the poem describes a statue maker making a statue. A metaphor is used because the sharp edges are not literally merging, or coming together. It just seems that way as the sculptor creates his statue. It looks as if the lines are literally coming together into smooth curves, when really the sculptor is gently tapping away.
If this were personification, it would be describing the lines as if they were human.