[git] Fix 'git-hub-shorten-url' helper

Changes:
- Use git.io over 'https'
- Add extra check to conform to *.github.com URLs
- Use built-in _urls function for completion
- Update readme with GitHub blog URL

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

@@ -8,4 +8,4 @@
 #   Sorin Ionescu <sorin.ionescu@gmail.com>
 #
 
-_arguments '1:url:' && return 0
+_arguments '::GitHub URL:_urls' && return 0

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

@@ -13,12 +13,12 @@ if [[ "$url" == '-' ]]; then
   read url <&0
 fi
 
-if [[ -z "$url" ]]; then
-  print "usage: $0 [ url | - ]" >&2
+if [[ -z "$url" || ! "$url" =~ ^https?:\/\/.*github.com\/ ]]; then
+  print "usage: $0 [ url | - ] ; must be a github.com URL" >&2
 fi
 
 if (( $+commands[curl] )); then
-  curl -s -i 'http://git.io' -F "url=$url" | sed -n 's/^Location: //p'
+  curl -s -i 'https://git.io' -F "url=$url" | sed -n 's/^Location: //p'
 else
   print "$0: command not found: curl" >&2
 fi