mentioned in MDL-28044 : http://tracker.moodle.org/browse/MDL-28044?focusedCommentId=115425&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-115425

Eloy said:
"3) And the most I think on it... the less I like (this applies to all branches, not only 19_STABLE) the include of version.php at all nor the parsing of $release to calculate $branch. After all, each branch has different code and we could have some constant/define with the 19/20... That would make everything really easy IMO.
.. about the 19_STABLE problem and the version.php consideration. Just imagine we invent version 10.1 or whatever, that will break the regexp, while having it in some constant will do things really easier and free of problems loading version.php multiple times or so."

it certainly makes more sense to define a specific branch variable to me. (sole purpose is for linking to docs initially?)