relnotes: add 6.6.3

relnotes/6.6

@@ -1,3 +1,12 b''
+= Mercurial 6.6.3 =
+
+ * tests: don't use "status" operand of dd in test-censor.t (issue6858)
+ * tests: make sha256line.py available for all tests
+ * tests: use sha256line.py instead of /dev/random in test-censor.t (issue6858)
+ * tests: fix nondeterministic test failure in test-contrib-perf.t
+ * rust-changelog: don't panic on empty file lists
+ * grep: restore usage of --include/--exclude options
+
 = Mercurial 6.6.2 =
 
  * histedit: remove superfluous echo() and endwin() calls (issue6859)