fixed bug with plugins not folowing master branch

[rosebeats]

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For plugins that are following a branch that is not master. (i.e. if you had Plug 'neoclide/coc.nvim', {'branch': 'release'}), outdated-plugins would incorrectly mark that plugin as not updated if the release branch was not caught up with master. This pull request fixes that by referencing the correct branch in the git rev-list command

[rosebeats]

I only just realized that there was actually already a pull request made to fix this bug. If you end up using that one, you can just close this as it does the same thing.