- the "r<revnum>" style has been made more strict; now it only accepts digits, no more hex digits.
- the "[<revnum>]" style still accepts hex digits, but the <revnum> has to have at least 8 digits (12 is common practice)
- the Subversion backend has now a restricted list of aliases for the latest changeset: only "head", "latest", "youngest" or the empty string are now accepted.
Fixes #4261.