I prefer the much simpler approach of reusing the existing utils with a simple shell script, which is much faster than ack:

http://www.pixelbeat.org/scripts/findrepo

A quick test on a moderately big repo:

    $ time findrepo test '*' | wc -l
    158819
    real	0m0.532s
    $ time ack -a test | wc -l
    76526
    real	0m8.762s