The lintCommit column in phabricator_repository.repository_branch is defined as VARCHAR(40). I assume this is based on the largest size of git or mercurial hashes. But with svn, the lintCommit column is populated with a value that includes the repository URI, which can grow arbitrarily longer than this.
I set my local copy to VARCHAR(255) and now the save_lint.php script is working without errors, but I don't know if this is enough to cover any reasonable case.