Update jgit to 5.1.16.202106041830-r
Update jgit to 5.1.16.202106041830-r

This update contains
- 8bc166b00 - BatchRefUpdate: Skip saving conflicting ref names and
  prefixes in memory
- 294a99af2 - BatchRefUpdateTest: Accurately assert RefsChangedEvent(s)
  fired
- 303dd019d - Optimize RefDirectory.isNameConflicting()

Change-Id: I570ddc05718290380fe1b3d3dcfb92ee18335b41

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