Fix bug: redisplay was needlessly recentering point whenever a change

Fix bug: redisplay was needlessly recentering point whenever a change
started at the first visible position of the buffer.  This is rarely
necessary.  This bug was deliberately introduced in revision 1.25 to
fix a more serious bug, in which the point could be left off-screen at
the end of redisplay.  This change fixes both bugs, by requesting
redisplay whenever a change might affect point's position; previously
it was incorrectly assumed that this request was unnecessary.