Change 'master@noon' syntax to 'master@{noon}'.

Change 'master@noon' syntax to 'master@{noon}'.

Its ambiguous to parse "master@2006-05-17 18:30:foo" when foo is
meant as a file name and ":30" is meant as 30 minutes past 6 pm.
Therefore all date specifications in a sha1 expression must now
appear within brackets and the ':' splitter used for the path name
in a sha1 expression ignores ':' appearing within brackets.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>