bash: teach __git_ps1 about REVERT_HEAD

diff --git a/RelNotes b/RelNotes
index 17d37af54951e3179282cad837ee0beb271eb4e9..7a99a15f2fd9141247b100eb9a5d0b9b88b0f2b9 120000 (symlink)
--- a/RelNotes
+++ b/RelNotes
@@ -1 +1 @@
-Documentation/RelNotes/1.7.2.5.txt
\ No newline at end of file
+Documentation/RelNotes/1.8.2.1.txt
\ No newline at end of file