revert: plug memory leak in "cherry-pick root commit" codepath

revert: plug memory leak in "cherry-pick root commit" codepath

The empty tree passed as common ancestor to merge_trees() when
cherry-picking a parentless commit is allocated on the heap and never
freed.  Leaking such a small one-time allocation is not a very big
problem, but now that "git cherry-pick" can cherry-pick multiple
commits it can start to add up.

Avoid the leak by storing the fake tree exactly once in the BSS
section (i.e., use a static).  While at it, let's add a test to make
sure cherry-picking multiple parentless commits continues to work.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>