git-gui: Make sure remotes are loaded when picking revisions

git-gui: Make sure remotes are loaded when picking revisions

If we are started for only a blame/browser/citool run we don't
usually initialize the list of remotes, or determine which refs
are tracking branches and which are local branch heads.  This is
because some of that work is relatively expensive and is usually
not going to be needed if we are started only for a blame, or to
make a single commit.

However by not loading the remote configuration we were crashing
if the user tried to open a browser for another branch through
the Repository menu, as our load_all_heads procedure was unable
to decide which refs/heads/ items were actually local heads.  We
now force all remote configuration data to be loaded if we have
not done so already and we are trying to create a revision mega
widget.

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