Upgrade JGit to 0.4.0-354-g21c3d82

This version of JGit should be safe to run a "git gc" or "git repack"
activity concurrently with a Gerrit Code Review daemon process, as
JGit can now recover when a pack disappears and its content has been
moved into another pack, or even back out to a loose object file.

Bug: GERRIT-81
Signed-off-by: Shawn O. Pearce <sop@google.com>