githooks: fix removal of branches

Removal of existing branch wasn't correctly handled. Merged branch
should be possible to remove but without this hook would complain that
it can't be removed because it is not based on top of master branch.

.githooks/pre-push

@@ -5,11 +5,6 @@ server_old="gitlab.labs.nic.cz"
 zero_sha1="0000000000000000000000000000000000000000"
 
 
-is_new_branch() {
-	local remote_sha1="$1"
-	[ "$remote_sha1" = "$zero_sha1" ]
-}
-
 compare_ancestors() {
 	local local_hash="$1"
 	[ "$local_hash" != "$zero_sha1" ] || return 0 # ignore removals
@@ -32,6 +27,11 @@ push_master=
 push_develop=
 
 while read -r local_ref local_sha1 remote_ref remote_sha1; do
+	if [ "$local_sha1" = "$zero_sha1" ]; then
+		# Always allow removal of branches
+		continue
+	fi
+
 	remote_ref="${remote_ref#refs/heads/}"
 	case "$remote_ref" in
 		master)
@@ -55,7 +55,7 @@ while read -r local_ref local_sha1 remote_ref remote_sha1; do
 		*)
 			# We terminate push only if this tries to create new branch of invalid
 			# name. This allows push to existing branches.
-			if is_new_branch "$remote_sha1"; then
+			if [ "$remote_sha1" = "$zero_sha1" ]; then
 				echo "Creation of new branch of this name is not allowed: $remote_ref" >&2
 				exit 1
 			fi