revisions.txt: escape "..." to avoid asciidoc horizontal ellipsis

In asciidoc's HTML output of the "gitrevisions" and "git-rev-parse"
documentation, the header:

  The ... (three-dot) Symmetric Difference Notation

is rendered using "&8230;", a horizontal ellipsis. This is visually
ugly, but also hard to search for or cut-and-paste. We really mean three
ascii dots (0x2e) here, so let's make sure it renders as such.

The simplest way to do that is just escaping the leading dot, as the
instances in the rest of the section do. Arguably this should all be
converted to use backticks, which would let us drop the quoting here and
elsewhere (e.g., {carat}). But that does change the rendering slightly.
So let's fix the bug first, and we can decide on migrating the whole
section separately.

Note that this produces an empty doc-diff of the manpages. Curiously,
asciidoc produces the same ellipsis entity in the XML file, but docbook
then converts it back into three literal dots for the roff output! So
the roff manpages have been correct all along (which may be a reason
nobody noticed this until now).

Reported-by: Arthur Milchior
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/revisions.txt

@@ -283,7 +283,7 @@ The '..' (two-dot) Range Notation::
  for commits that are reachable from r2 excluding those that are reachable
  from r1 by '{caret}r1 r2' and it can be written as 'r1..r2'.
 
-The '...' (three-dot) Symmetric Difference Notation::
+The '\...' (three-dot) Symmetric Difference Notation::
  A similar notation 'r1\...r2' is called symmetric difference
  of 'r1' and 'r2' and is defined as
  'r1 r2 --not $(git merge-base --all r1 r2)'.