Add option to allow empty commits to CommitCommand

Add option to allow empty commits to CommitCommand

CommitCommand should allow to specify whether empty commits (commits
having the same tree as the sole predecessor commit) are allowed or not.
Similar to native git's "--allow-empty" flag.

The defaults differ between JGit and native git even after this change.
When not specifying paths then by default JGit allows to create empty
commits while native git does not. It would be API breaking to change
this now.

Bug: 460301
Change-Id: I88feb0c3ffb2c686b1d0594e669729b065cda4cb
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>