git-gui: Rename all_branches -> all_heads.

Since this list is really the set of refs which match "refs/heads/*" it
really is the set of heads and not necessarily the set of all branches,
as the remote tracking branches are not listed in this set, even if it
appears in the "refs/heads/*" namespace (e.g. an old style repository).

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>