Mark COMMIT_GENERATION_* constants final

Mark COMMIT_GENERATION_* constants final

In org.eclipse.jgit.lib.Constants the constants are all marked final
with the exception of:

- COMMIT_GENERATION_UNKOWN
- COMMIT_GENERATION_NOT_COMPUTED

They were introduced by cf70e7cbe4f without the `final` keyword while
other constants have it which certainly has been forgotten.

The javadoc `{@value}` tag causes raises a warning about the fields not
being constants which is how I have discovered the ommission.

Change-Id: I0ad87f42355440c7d50158e773a280a0526e9671