Update JGit to the latest master revision
Update JGit to the latest master revision

This update includes the following commits:

91979f93f - TreeRevFilterTest: Simplify filter used
e4fd171da - reftable: remove assert calls
0902e060f - GpgKeyLocator: Return a signing key for a user instead of the master key
68b8317a0 - GpgKeyLocator: Return subkeys instead of the first key for subkeys
385b503ae - Support for core.hooksPath
b138f1694 - Upgrade Tycho to 1.5.0
c020848db - Close ObjectReaders in tests
123634e2a - Close some Repository instances in tests
1b8da0dcf - Close ObjectInserters in tests
ecc313a6f - Upgrade jacoco-maven-plugin to 0.8.5
c24eee4fa - [pgm] Add --mirror option to clone command
bceac7bd0 - Remove meaningless comment in CloneCommandTest
6216b0de8 - Implement mirror option in CloneCommand

There is nothing in particular that we need; this is only to keep
up to date with the head of the master branch.

Change-Id: I8c4df30b63704c6815edd671b3665906e3eb1b5b

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