Plumbing version of 'git branch --contains' ?

[Crabtree, Andrew]

I need to get a list of refs that can reach a certain SHA in in a script.

git branch --contains SHA 

would be great (runs in ~2 seconds), but not my preferred option for scripting.

I tried
 
for br in $(git for-each-ref --format='%(refname:short)' refs/heads/)
do
    git merge-base --is-ancestor $1 ${br}
    if [ $? -eq 0 ]
    then
            echo "${br}"
    fi
done

Which gives me perfect output, but takes 82 seconds to run in my environment.  

Is there an alternative I'm missing to give me the run time performance of 'git 
branch --contains' but with stable output suitable for parsing?

Thanks in advance,
-Andrew  
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