They can include slash / for hierarchical (directory) grouping, but
no slash-separated component can begin with a dot . or end with the
sequence .lock.
They must contain at least one /. This enforces the presence of a
category like heads/, tags/ etc. but the actual names are not
restricted. If the --allow-onelevel option is used, this rule is waived.
They cannot have two consecutive dots .. anywhere.
They cannot have ASCII control characters (i.e. bytes whose values
are lower than \040, or \177 DEL), space, tilde ~, caret ^, or colon :
anywhere.
They cannot have question-mark ?, asterisk *, or open bracket [
anywhere. See the --refspec-pattern option below for an exception to
this rule.
They cannot begin or end with a slash / or contain multiple
consecutive slashes (see the --normalize option below for an exception
to this rule)
They cannot end with a dot ..
They cannot contain a sequence @{. They cannot be the single character @.
They cannot contain a \.