completions: complete files where appropriate in bash

[Rylan12]

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This is a follow-up to #10229 which adds the ability to complete files where appropriate.

A few commands (bottle, ruby, sh, and style) accept files as named arguments. This PR will now enable automatic file completion for these commands. Note that this is only available in newer versions of bash, so I've added a conditional making the __brew_complete_files a no-op unless the compopt command exists. I've tested using builtin /bin/bash (no compopt command so no file completions) and Homebrew's /usr/local/bin/bash (with compopt and file completions).

[BrewTestBot]

Review period will end on 2021-01-20 at 04:58:03 UTC.

[MikeMcQuaid]

Nice work again @Rylan12!

Note that this is only available in newer versions of bash

Which versions, out of interest?

[Rylan12]

Which versions, out of interest?

The compopt command was added in version 4.0. Apple's built-in bash is verion 3.2

[BrewTestBot]

Review period ended.