bash: teach __git_ps1 about REVERT_HEAD

diff --git a/RelNotes b/RelNotes
index dfc0e738d9198a8b9a85ed3c8b1f621f71b32f25..7a99a15f2fd9141247b100eb9a5d0b9b88b0f2b9 120000 (symlink)
--- a/RelNotes
+++ b/RelNotes
@@ -1 +1 @@
-Documentation/RelNotes/1.7.8.5.txt
\ No newline at end of file
+Documentation/RelNotes/1.8.2.1.txt
\ No newline at end of file