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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[carlocab]

I don't think we want to pass this variable through transparently. Can we use USED_BY_HOMEBREW_VARS above instead?

You'll also need to make sure this interacts properly with HOMEBREW_SSH_CONFIG_PATH:

brew/Library/Homebrew/brew.sh

Lines 1093 to 1097 in c437bf7

	# Use this configuration file instead of ~/.ssh/config when fetching git over SSH.
	if [[ -n "${HOMEBREW_SSH_CONFIG_PATH}" ]]
	then
	export GIT_SSH_COMMAND="ssh -F${HOMEBREW_SSH_CONFIG_PATH}"
	fi

[sfc-gh-ksmogor]

I read the USED_BY_HOMEBREW_VARS and it looks fine to use this mechanism.

I also would like to understand how to treat the config and additional flags. Because from my understanding of the current code, it looks like -oBatchMode=yes and -F/config/path options are treated independently (as decorators). So it looks like the custom ssh command should be ready for decorating by those two options. Is it fine to assume that for brew users? Anyone wanted to use custom ssh command has to expect that brew will add some flags to it before passing it to git.

I can change all places that use plain ssh to fill GIT_SSH_COMMAND. Those places will use value from HOMEBREW_GIT_SSH_COMMAND or from global git config.

[MikeMcQuaid]

I read the USED_BY_HOMEBREW_VARS and it looks fine to use this mechanism.

To be more explicit: we're telling you we don't want to do that.

I can change all places that use plain ssh to fill GIT_SSH_COMMAND.

This seems overkill. Let's keep this change as tightly scoped as possible.

[MikeMcQuaid]

it is worth checking if this command is actually a non-empty string and valid?