Merge branch 'stable-5.1' into stable-5.2

Merge branch 'stable-5.1' into stable-5.2

* stable-5.1:
  Prepare 5.1.9-SNAPSHOT builds
  JGit v5.1.8.201906050907-r
  Test detecting modified packfiles
  Enhance fsTick() to use filesystem timer resolution
  Add debug trace to measure time needed to open pack index
  Extend FileSnapshot for packfiles to also use checksum to detect changes
  Wait opening new packfile until it can't be racy anymore
  Avoid null PackConfig in GC
  Add FileSnapshot test testing recognition of file size changes
  Capture reason for result of FileSnapshot#isModified
  Skip FileSnapshotTest#testSimulatePackfileReplacement on Windows
  Tune max heap size for tests
  Fix FileSnapshotTest.testNewFileNoWait() to match its javadoc
  ObjectDirectory: fix closing of obsolete packs
  Include filekey file attribute when comparing FileSnapshots
  Measure file timestamp resolution used in FileSnapshot
  Fix FileSnapshot's consideration of file size
  Fix API problem filters

Change-Id: I3ac77bfa03f7436de12ab86e1bba29afee5ccd01
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>