General support for ! in git-config values

alias.* values can start with !, in which case a shell command will be executed with the arguments you give.

For a program I'm working on (git-deploy) I'd like to have this as a general facility, i.e. users can specify either:

    foo.bar = value

Or:

    foo.bar = !cat /some/path

I'm wondering why git-config doesn't do this already, if there's no reason in particular I can just patch it in, either as a new option:

    git config --with-exec --get foo.bar

Or as the default behavior.