Document git rev-list --timestamp

Note that git log does not understand this option yet:

  $ git log --timestamp
  fatal: unrecognized argument: --timestamp
Signed-off-by: NJakub Narebski <jnareb@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-rev-list.txt

@@ -22,6 +22,7 @@ SYNOPSIS
 	     [ \--stdin ]
 	     [ \--topo-order ]
 	     [ \--parents ]
+	     [ \--timestamp ]
 	     [ \--left-right ]
 	     [ \--cherry-pick ]
 	     [ \--encoding[=<encoding>] ]
@@ -117,6 +118,9 @@ e.g. "2 hours ago".
 
 	Print the parents of the commit.
 
+--timestamp::
+	Print the raw commit timestamp.
+
 --left-right::
 
 	Mark which side of a symmetric diff a commit is reachable from.