bash: teach __git_ps1 about REVERT_HEAD

diff --git a/RelNotes b/RelNotes
index 5217e8e5543a696660e3f275d9717e0656d786f8..7a99a15f2fd9141247b100eb9a5d0b9b88b0f2b9 120000 (symlink)
--- a/RelNotes
+++ b/RelNotes
@@ -1 +1 @@
-Documentation/RelNotes-1.7.2.3.txt
\ No newline at end of file
+Documentation/RelNotes/1.8.2.1.txt
\ No newline at end of file