Upgrade JGit to 4.11.0.201803080745-r.2-g61e4f1665
Upgrade JGit to 4.11.0.201803080745-r.2-g61e4f1665

This is a snapshot build from jgit's stable-4.11 branch, and contains a
fixed version of ObjectIdSerializer [1] that supports serialization of
known non-null values.

[1] https://git.eclipse.org/r/#/c/119456/

Change-Id: I758be8a38e2aece932e145faf5e71867aa28713c

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