Use Homebrew Git's hunk headers

[garrettr]

Apple's Git and Homebrew's Git sometimes disagree about hunk headers. Homebrew's Git is newer and closer to other platform's versions of Git, so we prefer it.

After installing Homebrew's Git, I ran yarn run init and yarn run update_patches (without having made any changes to the Chromium source). There is one file with a hunk header change, because I resolved a merge conflict that was created in 9f292d21 because @bbondy was using Homebrew's Git while I was using Apple's Git.

I will update the wiki to recommend developers using macOS install git from Homebrew.

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.

Test Plan:

1. Make sure you're running a recent version of Git (e.g. 2.17.0). Don't use the version provided by Apple.
2. yarn run init
3. yarn run update_patches

There should be no bogus changes to the hunk headers in any of the patch files.

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions