revisions.txt: clarify the .. and ... syntax In <rev1>..<rev2> and <rev1>...<rev2>, if either <rev1> or <rev2> is omitted, it defaults to 'HEAD'. Add this detail to the document. Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>