formula_installer: ignore dependencies when fetching.

Whenever you are fetching a dependency you only care about fetching that
specific dependency and not all the dependencies of that dependency.

Unlike installing, dependencies can be fetched in any order and have no
"chains" between them.

Library/Homebrew/formula_installer.rb

@@ -572,6 +572,10 @@ class FormulaInstaller
     fi.verbose                 = verbose?
     fi.quiet                   = quiet?
     fi.debug                   = debug?
+    # When fetching we don't need to recurse the dependency tree as it's already
+    # been done for us in `compute_dependencies` and there's no requirement to
+    # fetch in a particular order.
+    fi.ignore_deps             = true
     fi.fetch
   end