Add REL9_1_STABLE to the set of branches tracked by git_changelog.

src/tools/git_changelog

@@ -36,9 +36,11 @@ require IPC::Open2;
 # Adjust this list when the set of interesting branches changes.
 # (We could get this from "git branches", but not worth the trouble.)
 # NB: master must be first!
-my @BRANCHES = qw(master REL9_0_STABLE REL8_4_STABLE REL8_3_STABLE
-    REL8_2_STABLE REL8_1_STABLE REL8_0_STABLE REL7_4_STABLE REL7_3_STABLE
-    REL7_2_STABLE REL7_1_STABLE REL7_0_PATCHES REL6_5_PATCHES REL6_4);
+my @BRANCHES = qw(master
+    REL9_1_STABLE REL9_0_STABLE
+    REL8_4_STABLE REL8_3_STABLE REL8_2_STABLE REL8_1_STABLE REL8_0_STABLE
+    REL7_4_STABLE REL7_3_STABLE REL7_2_STABLE REL7_1_STABLE REL7_0_PATCHES
+    REL6_5_PATCHES REL6_4);
 
 # Might want to make this parameter user-settable.
 my $timestamp_slop = 600;