Make git_remote_branch plugin git-flow friendly

This is an update to the git_remote_branch plugin by @paulmars.

In its current form it will incorrectly complete git-flow style branches, like `feature/foobar`, giving just `foobar`.

The plugin is getting a list of all remote branch and stripping off everything before the last `/` to get branch names. This commit changes that to only remove `<remote_name>/` at the start of a branch name. So `origin/feature/foobar` gets stripped to `feature/foobar`.