patch 8.2.3615: wrong indent in first line if re-formatting with indent expr

patch 8.2.3615: wrong indent in first line if re-formatting with indent expr

Problem:    When re-formatting with an indent expression the first line of a
            paragraph may get the wrong indent. (Martin F. Krafft)
Solution:   Apply the correct indenting function for the first line.
            (Christian Brabandt, closes #9150, closes #9056)