Qualify custom bash func name (#730)

* Qualify custom bash func name

 - fixes issue where multiple cobra apps using custom bash completion
 would have their __custom_func collide
 - support fallback to plain __custom_func to maintain compatibility

#694

* Improve tests for bash completion __custom_func

 - check for the correct number of occurrences of function name

#694

bash_completions.go

@@ -129,7 +129,13 @@ __%[1]s_handle_reply()
     fi
 
     if [[ ${#COMPREPLY[@]} -eq 0 ]]; then
-        declare -F __custom_func >/dev/null && __custom_func
+		if declare -F __%[1]s_custom_func >/dev/null; then
+			# try command name qualified custom func
+			__%[1]s_custom_func
+		else
+			# otherwise fall back to unqualified for compatibility
+			declare -F ___custom_func >/dev/null && __custom_func
+		fi
     fi
 
     # available in bash-completion >= 2, not always present on macOS