Bad formatting of multiline f-strings

[yutkin]

This code

str = f"this" \
      f"is a very" \
      f" long " \
      f"string."

print(str)

Black converts to

str = f"this" f"is a very" f" long " f"string."

print(str)

Why not just concatenate it to str = f"this is a very long string."?

black, version 18.6b3
Python 3.6.5

[zsol]

This is a dupe of #26, we're going to fix it!