brew-leaves: resolve dependency graph for installed formulae only

Closes Homebrew/homebrew#19870.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Contributions/cmd/brew-leaves.rb

@@ -4,10 +4,10 @@
 
 require 'formula'
 
-def get_used_by
+def get_used_by(formulae)
   used_by = {}
-  Formula.each do |f|
-    next if f.deps == nil
+  formulae.each do |f|
+    next if f.nil? or f.deps.nil?
 
     f.deps.each do |dep|
       _deps = used_by[dep.to_s] || []
@@ -19,8 +19,17 @@ def get_used_by
   return used_by
 end
 
-deps_graph = get_used_by()
 installed = HOMEBREW_CELLAR.children.select { |pn| pn.directory? }.collect { |pn| pn.basename.to_s }
+installed_formulae = installed.collect do |pn|
+  begin
+    Formula.factory(pn)
+  rescue FormulaUnavailableError
+    # Don't complain about directories from DIY installs
+  end
+end
+
+deps_graph = get_used_by(installed_formulae)
+
 installed.each do |name|
   deps = deps_graph[name] || []
   puts name unless deps.any? { |dep| installed.include? dep.to_s }