If the user has created (or deleted) a branch through an external tool,
and uses Rescan, they probably are trying to make git-gui update to show
their newly created branch.
So now we load all known heads and update the branch menu during any
rescan operation, just in-case the set of known branches was modified.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>