convert: change default for git rename detection to 50%
This default mirrors the default for 'git diff'. Other commands have slightly
different defaults -- for example, the move/copy detection for 'git blame'
assumes that a hunk is moved if more than 40 alphanumeric characters are the
same, or copied if more than 20 alphanumeric characters are the same. 50% seems
to be the most common default, though.