While the ref name encoding is unspecified, UTF-8 is preferred as
some output processing may assume ref names in UTF-8.
-'@'::
- '@' alone is a shortcut for 'HEAD'.
-
'<refname>@\{<date>\}', e.g. 'master@\{yesterday\}', 'HEAD@\{5 minutes ago\}'::
A ref followed by the suffix '@' with a date specification
enclosed in a brace