Move zsh completion logic to new subcommand: commit

Signed-off-by: Steve Durrheimer <s.durrheimer@gmail.com>

contrib/completion/zsh/_docker

@@ -526,7 +526,14 @@ __docker_container_subcommand() {
                 "($help -):containers:__docker_complete_running_containers" && ret=0
             ;;
         (commit)
-            __docker_subcommand && ret=0
+            _arguments $(__docker_arguments) \
+                $opts_help \
+                "($help -a --author)"{-a=,--author=}"[Author]:author: " \
+                "($help)*"{-c=,--change=}"[Apply Dockerfile instruction to the created image]:Dockerfile:_files" \
+                "($help -m --message)"{-m=,--message=}"[Commit message]:message: " \
+                "($help -p --pause)"{-p,--pause}"[Pause container during commit]" \
+                "($help -):container:__docker_complete_containers" \
+                "($help -): :__docker_repositories_with_tags" && ret=0
             ;;
         (cp)
             __docker_subcommand && ret=0
@@ -1672,14 +1679,7 @@ __docker_subcommand() {
                 "($help -):path or URL:_directories" && ret=0
             ;;
         (commit)
-            _arguments $(__docker_arguments) \
-                $opts_help \
-                "($help -a --author)"{-a=,--author=}"[Author]:author: " \
-                "($help)*"{-c=,--change=}"[Apply Dockerfile instruction to the created image]:Dockerfile:_files" \
-                "($help -m --message)"{-m=,--message=}"[Commit message]:message: " \
-                "($help -p --pause)"{-p,--pause}"[Pause container during commit]" \
-                "($help -):container:__docker_complete_containers" \
-                "($help -): :__docker_repositories_with_tags" && ret=0
+            __docker_container_subcommand && ret=0
             ;;
         (container)
             local curcontext="$curcontext" state