'+' increments the mtime on the files by <seconds>
'-' decrements the mtime on the files by <seconds>
'=' sets the mtime on the file to exactly <seconds>
'=+' and '=-' sets the mtime on the file to <seconds> after or
before the current time.
Signed-off-by: Eric Wong <normalperson@yhbt.net> Signed-off-by: Junio C Hamano <junkio@cox.net>