Properly set line properties when reflowing lines
The flags for a line that is the first in prompt/input/output block should only be for the first line.
- When splitting a long line to two lines, don't copy those flags.
- When merging two lines, use those flags from the first one.
This fixes bugs reported here: https://gitlab.gnome.org/GNOME/vte/-/issues/2681#note_1903991