git-check-ref-format: reject funny ref names.

Update check_ref_format() function to reject ref names that:

 * has a path component that begins with a ".", or
 * has a double dots "..", or
 * has ASCII control character, "~", "^", ":" or SP, anywhere, or
 * ends with a "/".

Use it in 'git-checkout -b', 'git-branch', and 'git-tag' to make sure
that newly created refs are well-formed.

Signed-off-by: Junio C Hamano <junkio@cox.net>

git-checkout.sh

@@ -17,6 +17,8 @@ while [ "$#" != "0" ]; do
 			die "git checkout: -b needs a branch name"
 		[ -e "$GIT_DIR/refs/heads/$newbranch" ] &&
 			die "git checkout: branch $newbranch already exists"
+		git-check-ref-format "heads/$newbranch" ||
+			die "we do not like '$newbranch' as a branch name."
 		;;
 	"-f")
 		force=1