git-gui: Optimize for newstyle refs/remotes layout

git-gui: Optimize for newstyle refs/remotes layout

Most people using Git 1.5.x and later are using the newer style
of remotes layout where all of their tracking branches are in
refs/remotes and refs/heads contains only the user's own local
branches.

In such a situation we can avoid calling is_tracking_branch
for each head we are considering because we know that all of
the heads must be local branches if no fetch option or Pull:
line maps a branch into that namespace.

If however any remote maps a remote branch into a local
tracking branch that resides in refs/heads we do exactly
what we did before, which requires scanning through all
fetch lines in case any patterns are matched.

I also switched some regexp/regsub calls to string match
as this can be a faster operation for prefix matching.

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