Refresh installed list before dependency check

[muneebmahmed]

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Possibly fixes #8997. I call FormulaInstaller.installed.to_a after upgrading dependents, in order to refresh the list of formulae before checking for broken dependents. With this change, I ran brew upgrade x264 and both x264 and its dependent ffmpeg were upgraded without errors. When I checked out master, brew upgrade ruby ran into errors when upgrading its dependent cocoapods.

I did run into one error when running brew tests:

  1) Utils::Git#cherry_pick! aborts when cherry picking an existing hash
     Failure/Error:
       expect {
         described_class.cherry_pick!(HOMEBREW_CACHE, file_hash1)
       }.to raise_error(ErrorDuringExecution, /Merge conflict in README.md/)

       expected ErrorDuringExecution with message matching /Merge conflict in README.md/, got #<ErrorDuringExecution: Failure while executing; `/usr/local/Homebrew/Library/Homebrew/shims/scm/git -C /tmp/homebrew-tests-20201026-44218-1ytlg8a/cache cherry-pick 36c62d2` exited with 0. Here's the output:

However, I got this exact same error when running brew tests on master

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Possibly fixes Homebrew/homebrew-core#63497 as well

[MikeMcQuaid]

Thanks so much for your first contribution! Without people like you submitting PRs we couldn't run this project. You rock, @muneebmahmed!