6aa7b47ae1
Since #4748 `HOMEBREW_GIT` is set by the `brew.sh`, whose value is default to be `git`. As a result, it completely bypasses the logic of the shims/scm/git. This fixes the issue by checking whether `HOMEBREW_GIT` and `HOMEBREW_SVN` are set to be `git` and `svn` respectively. Fixes #4825.