git-gui: Correctly categorize tracking branches and heads.
Up until now git-gui did not support the new wildcard syntax used to
fetch any remote branch into a tracking branch during 'git fetch'. Now
if we identify a tracking branch as ending with the string '/*' then
we use for-each-ref to print out the reference names which may have
been fetched by that pattern. We also now correctly filter any
tracking branches out of refs/heads, if they user has placed any there.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>