envsetup: cmgerrit: fix for zsh

zsh does not define $FUNCNAME, define it for zsh when necessary.

Change-Id: I097d3e572b18c84d953ca03b3714271d1d2fd5c3
diff --git a/envsetup.sh b/envsetup.sh
index 35dc8e1..eeb6534 100644
--- a/envsetup.sh
+++ b/envsetup.sh
@@ -1831,6 +1831,12 @@
 }
 
 function cmgerrit() {
+
+    if [ "$(__detect_shell)" == "zsh" ]; then
+        # zsh does not define FUNCNAME, derive from funcstack
+        local FUNCNAME=$funcstack[1]
+    fi
+
     if [ $# -eq 0 ]; then
         $FUNCNAME help
         return 1