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.
This is the (partially) documented public API for Homebrew.
The main class you should look at is the {Formula} class (and classes linked from there). That's the class that's used to create Homebrew formulae (i.e. package descriptions). Assume anything else you stumble upon is private.