WorkTreeUpdater: re-format and clean-up

WorkTreeUpdater: re-format and clean-up

Reformat using the standard JGit formatter settings. Clean-ups:

* Try to improve javadoc.
* Remove blindly copy-pasted "@since 6.1" annotations.
* Get rid of private method nonNullNonBareRepo(); it's not needed.
* Simplify method nonNullRepo(), and annotate as @NonNull.
* Rename setInCoreFileSizeLimit() to getInCoreFileSizeLimit().

Change-Id: Ib1797e7cf925d87554307468330971e8ab2e05e9
Signed-off-by: Thomas Wolf <twolf@apache.org>
Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>