Documentation: do not convert ... operator to ellipses

Documentation: do not convert ... operator to ellipses

The symmetric difference or merge-base operator ... as used by
rev-list and diff is actually three period characters.  If it
gets replaced by an ellipsis glyph in the manual, that would
stop readers from copying and pasting it.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>