[git] Support short-code and improve completion in 'git-hub-shorten-url'

Changes:
- Add optional short-code support
- Improve completion for github.com URL (`http(s)://*.github.com` only)
- Return with non-zero exit code appropriately

modules/git/functions/_git-hub-shorten-url

@@ -8,4 +8,9 @@
 #   Sorin Ionescu <sorin.ionescu@gmail.com>
 #
 
-_arguments '::GitHub URL:_urls' && return 0
+local service="$service"
+
+zstyle ":completion:*:${service}:*:prefixes" ignored-patterns '^http(|s)://'
+zstyle ":completion:*:${service}:*:hosts" ignored-patterns '^*github.com'
+
+_arguments '1::GitHub URL:_urls' '2::code:' && return 0