add --branch

acme.sh

@@ -5515,6 +5515,7 @@ Parameters:
   --openssl-bin                     Specifies a custom openssl bin location.
   --use-wget                        Force to use wget, if you have both curl and wget installed.
   --yes-I-know-dns-manual-mode-enough-go-ahead-please  Force to use dns manual mode: $_DNS_MANUAL_WIKI
+  --branch, -b                      Only valid for '--upgrade' command, specifies the branch name to upgrade to.
   "
 }
 
@@ -6059,6 +6060,10 @@ _process() {
         _use_wget="1"
         ACME_USE_WGET="1"
         ;;
+      --branch | -b)
+        export BRANCH="$2"
+        shift
+        ;;
       *)
         _err "Unknown parameter : $1"
         return 1