Change default similarity score to 50(%) to match git's default

Git uses a default similarity score of 50(%) for rename detection.
Fix RenameDetector to use the same default score.

See https://git-scm.com/docs/git-diff#Documentation/git-diff.txt--Mltngt

Bug: jgit-110
Change-Id: I4b75910a02bca1afc108ad9e5609fda1e49a29da