Howto get the merge-base ?

Hi,

I'm trying to know which commit it the parent of a merge.
For exemple if I do that :

   o Merge
  / \
 /   \
 |   |
 |   o Commit D
 |   |
 |   o Commit C
 |   |
 o   | Commit B
 \  /
  \/
  o Commit A
  |
  o Init

How could I know that ``Commit A'' is the merge-base of ``Merge'' ?

I try to get this git-merge-base but result is strange and quiet
mysterious as he return me always second args I passed to. I'm using
git 1.3.2

-- 
Beber
#e.fr@freenode