git-gui: Fix Blame Parent & Context for working copy lines.

Make Blame Parent Commit and Show History Context work
properly for lines blamed on the working copy.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Shawn O. Pearce <sop@google.com>
diff --git a/git-gui.sh b/git-gui.sh
index 10d8a44..9fbee24 100755
--- a/git-gui.sh
+++ b/git-gui.sh
@@ -1025,6 +1025,9 @@
 set is_3way_diff 0
 set selected_commit_type new
 
+set nullid "0000000000000000000000000000000000000000"
+set nullid2 "0000000000000000000000000000000000000001"
+
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