Allow git.current_branch to work with git<1.7.8

"git branch" and "git branch --list" are functionally identical, but the
"--list" parameter was not added to git until version 1.7.8, so older
versions of git (such as the one in the RHEL/CentOS 6 repositories) are
unable to successfully run this function.

This commit removes the "--list" from the git command used in this
function.

Fixes #9468.