Git tree/blob and corresponding commit

Something I'm trying to figure out and can't find the answer myself. I'm 
browsing a tree, for example:

    $ git ls-tree HEAD~100

This list a set of files with the corresponding blob. Each blob has it's 
own SHA1. Fine.

Now given a SHA1 I'd like to know which commit has created (was 
containing) this specific blob?

What is the best way to achieve that?

Thanks.

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--| Pascal Obry                           Team-Ada Member
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