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ARROW-7949: [Git] Ignore macOS specific file: 'Brewfile.lock.json'

In the developer guides for Python, there is a suggestion for users on
macOS to use Homebrew to install all dependencies required for building
Arrow C++. This creates a 'cpp/Brewfile.lock.json' file is specific to
the system it sits on.

It would be desirable for this not to be tracked by version control. To
prevent this accidental addition, perhaps it should be ignored in the
gitignore file for the repository

Fixes [#ARROW-7949](https://issues.apache.org/jira/browse/ARROW-7949)

REF:
https://arrow.apache.org/docs/developers/python.html#building-on-linux-and-macos

	modified:   .gitignore

Closes #6494 from tallamjr/issue/7949/gitignore and squashes the following commits:

7cb416746 <Tarek Allam> Ignore macOS specific file: 'Brewfile.lock.json'

Authored-by: Tarek Allam <t.allam.jr@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>