Eclipse JGit: Java implementation of Git 5.1.16

Performance Improvements

• BatchRefUpdate: Skip saving conflicting ref names and prefixes in memory. This change improves the complexity of creating n refs using BatchRefUpdate from O(n) to O(1).
• Optimize RefDirectory.isNameConflicting(). This change improves complexity of creating one ref when n loose refs are present from O(n) to O(1).