download_strategy: fix Git repo updating.

Git repositories should always be updated when the ref is a branch.
They should be also updated if no ref was specified or if the ref isn’t
present. This stops the repo being updated if we’re building a specific
revision and it is present. Additionally, this stops the update barfing
on repositories where we’re using a revision and there is no upstream
`master` branch.

Closes Homebrew/homebrew#29218.

Library/Homebrew/download_strategy.rb

@@ -570,7 +570,9 @@ class GitDownloadStrategy < VCSDownloadStrategy
   end
 
   def update_repo
-    unless @ref_type == :tag && has_ref?
+    # Branches always need updated. The has_ref? check will only work if a ref
+    # has been specified; if there isn't one we always want an update.
+    if @ref_type == :branch || !@ref || !has_ref?
       quiet_safe_system 'git', 'fetch', 'origin'
     end
   end