api: {branch} placeholder in gh api calls should not inherit from the local repo if $GH_REPO is set #7603

Describe the bug

The api command forces the use of cmdutil.OverrideBaseRepoFunc(), which manually sets the owner and repo to the specified github repository.

If the user is 'in' an existing repository on the cli and calls the command with {branch} as a placeholder, it fills that placeholder with the name of the checked-out-branch from the cwd.

This feels like it should not be the behaviour, as the usage text suggests that all three valid placeholders ['owner', 'repo', 'branch'] will be obtained from $GH_REPO if it is set.

Steps to reproduce the behavior

1. cd /path/to/some/git/repo/on/github
2. git checkout -b wip/tw/poc
3. GH_REPO=cli/cli gh api '/some/call/{owner}/{repo}/{branch}'

Expected vs actual behavior

Expected

Either:

1. A request is made to the api at /some/call/cli/cli/null
2. Some error is thrown as we have no good value for {branch} under these conditions

Observed

A request is made to the api at /some/call/cli/cli/wip/tw/poc