utils/popen: add safe argument to popen_read and popen_write

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Follow-up to #10245

This PR adds a safe argument to popen_read and popen_write that can then be used in Utils.git_head

The reason for adding this is that formulae use safe_popen_read to get the git head, while Utils.git_head currently uses popen_read. Since Utils.git_head should primarily be used by formulae, this PR adds a safe argument to git_head that has a default value of true (whereas the default value is false for popen_read)

I also added tests for safe_popen_read and safe_popen_write which passed with the original methods as seen in the test runs for commit 40d85ec: https://github.com/Homebrew/brew/runs/1689977863

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Review period will end on 2021-01-13 at 17:55:14 UTC.

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Review period ended.