git-gui: more issues with diff parsing

[Michele Ballabio <barra_cuda@katamail.com>]

The patch
	git-gui: Fix diff parsing for lines starting with "--" or "++"
seems to have introduced some glitches. With this sequence:

git init
touch g
git add g
git commit -m"g is a file"
rm g
echo "vvvv" > file
ln -s file g
git add g file
git gui

Now clicking on "g" in the staged changes, git-gui gives this line:
	error: Unhandled 2 way diff marker: {d}

The following patch seems to fix this particular issue, but I don't think
it's the right fix...

diff --git a/lib/diff.tcl b/lib/diff.tcl
index a30c80a..0dac732 100644
--- a/lib/diff.tcl
+++ b/lib/diff.tcl
@@ -345,6 +345,8 @@ proc read_diff {fd scroll_pos} {
 				set tags {}
 			}
 			}
+		} elseif [string match {diff --git *} $line] {
+			continue
 		} else {
 			set op [string index $line 0]
 			switch -- $op {