Improve create branch REST API error message for ref names that are too long

When using the REST API to create a branch, if a branch name that is too long is supplied, the error message states that the maximum length of a fully qualified ref name is 111 characters. This could be confusing if the supplied branch name is not a fully qualified ref, but is between 100 and 111 characters long. This is because the branch name is limited to 100 characters, which makes the total limit 111 characters because of the prefix.

Suggested improvement:

• If a fully qualified ref name is supplied to the REST API, note that the maximum length is 111 chars
• If a non-qualified ref name is supplied, note that the maximum of an unprefixed ref name is 100 chars with a maximum of 111 for the fully qualified name