formula_installer: check if dependencies have already been fetched.

Given a `pkg_a` that depends on `pkg_b`, it is redundant to list
`pkg_b` as a dependency when running the following commands:

- `brew install pkg_b pkg_a`
- `brew upgrade` (when both `pkg_a` and `pkg_b` are outdated)

Library/Homebrew/formula_installer.rb

@@ -1156,7 +1156,10 @@ class FormulaInstaller
   def fetch_dependencies
     return if ignore_deps?
 
-    deps = compute_dependencies
+    deps = compute_dependencies.reject do |dep, _options|
+      self.class.fetched.include?(dep.to_formula)
+    end
+
     return if deps.empty?
 
     oh1 "Fetching dependencies for #{formula.full_name}: " \