Junio C Hamano, Fri 2013-07-19 @ 09:48:06-0700:
> But there is a very commonly accepted long tradition for "-" to mean
> "read from the standard input", so we cannot reuse it to mean "the
> branch I was previously on" for every command without first making
> sure the command will never want to use "-" for the other common
> purpose.

It may be worth noting that Bash (and probably other shells as well)
allow you to type `cd -` to switch back to the previous working
directory. I always (apparently mistakenly) assumed that
`git checkout -` was deliberately designed to mirror that shortcut. I
think the symmetry gives it some credibility, in any case. This
alternative meaning for - is not totally without precedent.

What would it mean to check out the standard input, anyway? I cannot see
how that could ever make sense, unless `git checkout` gains some
additional capaabilities that are unrelated to its current purpose.
Maybe I am just being myopic, though.