`/usr/bin/<tool>` will be a popup stub under such configuration.

The idea is to let `scm/git` to handle all of git location resolution
throughout Homebrew codebase.

We cannot really recover from this, but at least we'll provide a clearer
error message than trying to call `exec` with an empty string.

Some things can break in hard to debug ways if users or build scripts
set these variables, causing our otherwise quite robust Ruby wrappers
to fail. In theory, we could also use `--disable-rubyopt`, but this is
not supported in Ruby 1.8 (and we still care about it).

Use the same interpreter for the shims that is also used to run the main
Homebrew process (the one invoked via `brew`). The magic basically lies
in executing `ruby` with the `-x` option (supported since at least 1.8)
and in the following shebang line.

Closes Homebrew/homebrew#42354.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

This strategy is only used in one formula, so we can just replace it
with a dependency. It could be done with a requirement, but since this
is a low-impact change (it only matters for head builds on Leopard), I'm
not sure it's worth the effort.

The constant is retained for compatibility, though I could find only one
other reference to it on GitHub, in a formula that was removed from core
almost two years ago.

Closes Homebrew/homebrew#32615.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>