Rely on the implicit git branch list mode rather than the newer
explicit --list flag that doesn't work on git<1.7.8

"git branch --list" was added in Git v1.7.8 and doesn't work in older
versions that are still present on rhel<=6 and derivatives (such as
centos) and possibly more distributions.

The simpler "git branch" is equivalent and works identically on older
and newer versions of git.