Use a PTY when using external diff command from git config

[brandondong]

PR Description

I hit the same issue as #3119 when setting useExtDiffGitConfig to true and using difftastic side-by-side comparison in my git config.

The same fix in #3120 needs to be applied for the newer config option.

Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc

[stefanhaller]

Thanks, looks good.