grouping, but no slash-separated component can begin with a
dot `.`.
+. They must contain at least one `/`. This enforces the presence of a
+ category like `heads/`, `tags/` etc. but the actual names are not
+ restricted.
+
. They cannot have two consecutive dots `..` anywhere.
. They cannot have ASCII control characters (i.e. bytes whose
. They cannot contain a sequence `@{`.
+- They cannot contain a `\\`.
+
These rules make it easy for shell script based tools to parse
reference names, pathname expansion by the shell when a reference name is used
unquoted (by mistake), and also avoids ambiguities in certain