We are using git polling with a specific branch, for example we configure git://git.foo.nl/atcd.git as repository URL and refs/heads/master as branch to build. For the polling we see that the following command is used:
git -c core.askpass=true ls-remote -h git://git.foo.nl/atcd.git

Which returns all heads, it would be more optimal when the following is used, that only returns 1 branch
git -c core.askpass=true ls-remote -h git://git.foo.nl/atcd.git refs/heads/master