applypatch: no need to do non-portable [[ ... ]]

... when old, proven, case would do.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

git-applypatch.sh

@@ -14,7 +14,7 @@
 USAGE='<msg> <patch> <info> [<signoff>]'
 . git-sh-setup
 
-[[ "$#" = "3" || "$#" = "4" ]] || usage
+case "$#" in 3|4) usage ;; esac
 
 final=.dotest/final-commit
 ##