Fix "jgit checkout -f" to overwrite dirty worktree files

Fix "jgit checkout -f" to overwrite dirty worktree files

CheckoutCommand had a setForce() method. But this didn't correspond
to native git's 'git checkout -f' option. Deprecate the old setForce()
method and move its implementation to a new method setForceRefUpdate()
and use it to implement the -B option in the CLI class Checkout.

Add a setForced() method and use it to fix the associated '-f' option of
the CLI Checkout class to behave like native git's 'git checkout -f'
which overwrites dirty worktree files during checkout.

This is still not fully matching native git's behavior: updating
additionally dirty index entries is not done yet.

Bug: 530771
Change-Id: I776b78eb623b6ea0aca42f681788f2e4b1667f15
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>