Bump JGit to 3a7a9cb0e (stable-6.10 tip)

This adds support for the 'union' merge conflict strategy via
gitattribute configuration. Many Gerrit APIs do not load gitattribute
data, so a follow-up change is necessary to enable it.

$ git log --oneline --no-merges 1cd87ab79...3a7a9cb0e
3a7a9cb0e ResolveMerger: Allow setting the TreeWalk AttributesNodeProvider
4f48a5b1e Add Union merge strategy support
eeccc5a92 Update Apache sshd to 2.14.0
ed0835856 LfsConnectionFactoryTest: remove unnecessary cast
72652ff65 JSchSshProtocol2Test: remove unnecessary cast
a2e477296 ApacheSshProtocol2Test: remove unnecessary cast
9e1cd8aec NoteMapMerger: remove unnecessary cast

Change-Id: I4da8675847260697989e0a692e4a60dac310bae6
Release-Notes: Bump JGit to 3a7a9cb0e for Union merge conflict strategy support