git-gui: show unstaged symlinks in diff viewer

git-gui: show unstaged symlinks in diff viewer

git-gui has a minor problem with regards to symlinks that point
to directories.

	git init
	mkdir realdir
	ln -s realdir linkdir
	git gui

Now clicking on file names in the "unstaged changes" window,
there's a problem coming from the "linkdir" symlink: git-gui
complains with

	error reading "file4": illegal operation on a directory

...even though git-gui can add that same symlink to the index just
fine.

This patch fix this by adding a check.

[sp: Minor fix to use {link} instead of "link" in condition
     and to only open the path if it is not a symlink.]

Signed-off-by: Michele Ballabio <barra_cuda@katamail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>