How to check SVN repository version

cat svn-repo/format/db

If the file contents lists 3 on the first line, it stands for SVN 1.5, 4 for SVN 1.6
Credits: http://stackoverflow.com/a/282484/129497

The repository can be upgraded with:

svnadmin upgrade svn-repo

This can be important because SVN 1.5 and 1.6 introduced many improvements to branch operations including the svn mergeinfo command. If the repository is in an older version there is not enough metadata to ease the branch synchronizations.