rebase: give a better error message for bogus branch

diff --git a/RelNotes b/RelNotes
index 2dad8b277db3a95919bd904133d7e7cc3e323cb9..39dc075ee01342db1cd11c47f6de0395df605976 120000 (symlink)
--- a/RelNotes
+++ b/RelNotes
@@ -1 +1 @@
-Documentation/RelNotes-1.6.4.4.txt
\ No newline at end of file
+Documentation/RelNotes/1.7.3.5.txt
\ No newline at end of file