convert: ignore svn:executable for subversion targets without exec bit support Calling propset/propdel with subversion 1.6 on FAT gave abort: svn exited with status 256 and made test-convert-hg-svn.t and test-convert-svn-sink.t fail. 1.7 worked. This is a rework of fdee5d614fcc but ignores the executable bit when it isn't supported instead of using an approximation.