merge-base: "--is-ancestor A B"

In many scripted Porcelain commands, we find this idiom:

    if test "$(git rev-parse --verify A)" = "$(git merge-base A B)"
    then
    	... A is an ancestor of B ...
    fi

But you do not have to compute exact merge-base only to see if A is
an ancestor of B.  Give them a more direct way to use the underlying
machinery.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

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