Merge branch 'stable-6.4'

* stable-6.4:
  If tryLock fails to get the lock another gc has it
  Fix GcConcurrentTest#testInterruptGc
  Don't swallow IOException in GC.PidLock#lock
  Check if FileLock is valid before using or releasing it

Change-Id: Ia2797b44a60342eb9df53f0b3d674cba92a512fc

diff --cc org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/GC.java
Simple merge