completion/git_flow: use _command_exists

completion/available/git_flow.completion.bash

@@ -172,6 +172,7 @@ __git_flow_list_hotfixes ()
 }
 
 # temporarily wrap __git_find_on_cmdline() for backwards compatibility
-if [ -z "`type -t __git_find_subcommand`" ]; then
+if ! _command_exists __git_find_subcommand
+then
 	alias __git_find_subcommand=__git_find_on_cmdline
 fi