Adjust update to honor custom SSH command

[sfc-gh-ksmogor]

In my use case I use custom ssh command to access homebrew repositories (set as a global config in ~/.gitconfig file). Current brew update doesn't work with my custom repository because of wrong credentials. Tapping still works as expected.
It turned out that setting up explicit GIT_SSH_COMMAND=${GIT_SSH_COMMAND:-ssh} -oBatchMode=yes overrides global git config. Additionally, I couldn't override the default ssh command by exporting GIT_SSH_COMMAND. It happened because the flag wasn't whitelisted.

I propose to fix two things:

1. Add GIT_SSH_COMMAND to whitelisted list of envs. Currently passing this flag is dead logic without whitelisting.
2. Check whether the git config have custom ssh command. If so, doesn't override it. Use -oBatchMode only when default ssh implementation is used. Otherwise brew can break custom wrapper by adding ssh-only flag.

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• AI was used to generate or assist with generating this PR. Please specify below how you used AI to help you, and what steps you have taken to manually verify the changes.
  Used for searching the code through and diagnosing the problem with passing the flag to update (comparison between tap and update, suggesting the change when problem was pinned down by local modification).

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