JGit: Adapt to the split of jsch code in own artifact

Since: [1] and corresponding issue: [2] JCraft JSch code is extracted in
its own artifact: @jgit//org.eclipse.jgit.ssh.jsch:ssh-jsch. Adapt the
code correspondingly to that split.

This change is updating JGit module. The change we need:

8d2d68365 Decouple JSch from JGit Core

[1] https://git.eclipse.org/r/#/c/156153
[2] https://bugs.eclipse.org/bugs/show_bug.cgi?id=553625

Change-Id: I4a1f5f4ffbbf07b4455780c1b82b8d9005377585

plugins/BUILD

@@ -71,6 +71,7 @@ EXPORTS = [
     "//lib/jackson:jackson-core",
     "//lib:jgit-servlet",
     "//lib:jgit",
+    "//lib:jgit-ssh-jsch",
     "//lib:jsr305",
     "//lib/log:api",
     "//lib/log:log4j",