Fix vim lag/ghost-characters bug.

Fix:
	-vim suddenly became laggy, and occasionally spewed characters onto the
	screen whenever the cursor was moved.
	-track down issue to the statusline's GitBranchName() call, which made a
	system call to determine the git branch name of the open file. Call was made
	everytime the statusline updated, hence generating lag and ghost characters
	(presumably printed from the terminal).
	-fix by caching the branch name in a variable.