Rename segment vcs_branch -> branch

The segment names are now consistent with the ones in
powerline.segments.common .
Missed one more rename vcs_branch -> branch.
Add a spacer segment for getting the correct colors.

When using powerline with tmux, if there is data rendered before the
first segment directly using tmux's status option, this spacer
segment can be used to fill up as a dummy segment which adds the desired
colored arrow.
Adjust segment colors for tmux termsaber theme.