Merge branch 'db/length-as-hash' (early part) into db/svn-fe-code-purge

* 'db/length-as-hash' (early part): vcs-svn: implement perfect hash for top-level keys vcs-svn: implement perfect hash for node-prop keys vcs-svn: improve reporting of input errors vcs-svn: make buffer_copy_bytes return length read vcs-svn: make buffer_skip_bytes return length read vcs-svn: improve support for reading large files Conflicts: vcs-svn/fast_export.c vcs-svn/svndump.c