How do I qualify paths in the .gitignore file w.r.t. the repo root directory?

[Brent Goodrick <bgoodr@gmail.com>]

Say I have these files and directories [2]:

  /home/smart_tator/misc_files/.gitignore
  /home/smart_tator/misc_files/foo/
  /home/smart_tator/misc_files/bar/
  /home/smart_tator/misc_files/bar/baz/foo/
  /home/smart_tator/misc_files/bar/baz/real/

then I do:

  cd /home/smart_tator/misc_files/; git init

and say I have this line in that .gitignore file:

  foo/

And then I naively execute:

  git add bar/

then the bar/baz/real/ is added, but these are dutifully ignored:

  /home/smart_tator/misc_files/foo/
  /home/smart_tator/misc_files/bar/baz/foo/

But consider in my real repo, I have thousands of files, versus the
Tinker Toy example shown above. And consider that I don't know about
that bar/baz/foo/ exists ahead of time, because I'm not the only
developer checking in content to the repo that might contain precious
"foo/"'s to keep. I can't move my top level foo/, as I have tools that
rely upon that foo/ being in its place.

That means that we can't solve this problem efficiently/effectively
with the negation operator in the .gitignore file:

  foo/
  !bar/baz/foo/

because sooner or later someone is going to get surprised as to why
their foo/ isn't being added in some other subdirectory, and they will
"fix it" by committing a change where they have removed the foo/ from
the .gitignore file, thus causing the top level foo/ to show up as
candidates for addition in git status output.

Is there some way to express that the foo/ that is to be ignored is
always the one in /home/smart_tator/misc_files/ directory, but all
other foo/ subdirectories in any other directory under consideration
should still continue to participate in adds and merges, all without
having to "over-express" the exceptions with negation operators?

Maybe the imaginary .gitignore syntax I am thinking of would be one of
the following (most of these are just silly/fun):

  </>foo/  # <-- Huh?
  <top>foo/  # <-- What the ...?
  //foo/  # <-- Smells like Perforce or Windows UNC paths
  /foo/ # <-- No! Matches UNIX root filesystem directory paths!
  >foo/  # <-- You can't have a > character in DOS or Unix paths, can you?
  $root/foo/  # <-- I like this syntax the best [1]

Thanks,
bg

[1] Maybe there are other "variables" besides $root that might be
    useful to be added in the future, like $HOME.
[2] By "repo root directory", I mean whatever "$GIT_DIR/.." resolves to.