Support cherry-picking a root commit

Support cherry-picking a root commit

Handle the case of the commit to be picked not having any parents.

Since JGit implements cherry-pick as a 3-way-merge between the commit
to be picked and the target commit, using the parent of the picked
commit as merge base, this is super simple: just don't set a base tree.
The merger will not find any merge base and will supply an empty tree
iterator for the base.

Bug: 581832
Change-Id: I88985f1b1723db5b35ce58bf228bc48d23d6fca3
Signed-off-by: Thomas Wolf <twolf@apache.org>