Pass empty file to p4merge where no base is suitable.

diff --git a/RelNotes b/RelNotes
index 9148d7c5c2d201bf9ad28a3487be0aeb7075721e..540b756d2f64d8c8ebf927a707b9b2cc0a4c6151 120000 (symlink)
--- a/RelNotes
+++ b/RelNotes
@@ -1 +1 @@
-Documentation/RelNotes-1.7.0.4.txt
\ No newline at end of file
+Documentation/RelNotes/1.7.5.txt
\ No newline at end of file