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>

lib/diff.tcl

@@ -85,11 +85,16 @@ proc show_diff {path w {lno {}}} {
 	if {$m eq {_O}} {
 		set max_sz [expr {128 * 1024}]
 		if {[catch {
-				set fd [open $path r]
-				fconfigure $fd -eofchar {}
-				set content [read $fd $max_sz]
-				close $fd
-				set sz [file size $path]
+				if {[file type $path] == {link}} {
+					set content [file readlink $path]
+					set sz [string length $content]
+				} else {
+					set fd [open $path r]
+					fconfigure $fd -eofchar {}
+					set content [read $fd $max_sz]
+					close $fd
+					set sz [file size $path]
+				}
 			} err ]} {
 			set diff_active 0
 			unlock_index