Merge branch 'stable-5.6' into stable-5.7

# By Matthias Sohn (3) and Han-Wen Nienhuys (1)
* stable-5.6:
  Update API problem filter
  Prepare 5.6.2-SNAPSHOT builds
  JGit v5.6.1.202002131546-r
  Simplify ReftableCompactor

Change-Id: I16ed174f9fc662934c3ebaea85a60690efbed1c6
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

diff --cc org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/reftable/ReftableCompactorTest.java
Simple merge

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

diff --cc org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/MergedReftable.java
Simple merge

diff --cc org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/Reftable.java
Simple merge

diff --cc org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/ReftableCompactor.java
Simple merge

diff --cc org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/ReftableReader.java
Simple merge