unindent: ensure that a partial line gets displayed properly afterwards

When only the trailing chunk of a line is displayed on the top row of
the screen, and the unindenting of this line leads to a reduction in
the number of chunks, then the starting point of the viewport needs
to be re-evaluated.  For simplicity, do this always when something is
unindented.

This fixes https://savannah.gnu.org/bugs/?56102.
Reported-by: Devin Hussey <husseydevin@gmail.com>

Bug existed since around version 2.9.2.

src/text.c

@@ -262,6 +262,7 @@ void do_unindent(void)
 	}
 
 	set_modified();
+	ensure_firstcolumn_is_aligned();
 	refresh_needed = TRUE;
 	shift_held = TRUE;
 }