git-checkout: improve error messages, detect ambiguities.

git-checkout: improve error messages, detect ambiguities.

The patch is twofold: it moves the option consistency checks just under
the parse_options call so that it doesn't get in the way of the tree
reference vs. pathspecs desambiguation.

The other part rewrites the way to understand arguments so that when
git-checkout fails it does with an understandable message. Compared to the
previous behavior we now have:

  - a better error message when doing:

        git checkout <blob reference> --

    now complains about the reference not pointing to a tree, instead of
    things like:

        error: pathspec <blob reference> did not match any file(s) known to git.
        error: pathspec '--' did not match any file(s) known to git.

  - a better error message when doing:

        git checkout <path> --

    It now complains about <path> not being a reference instead of the
    completely obscure:

        error: pathspec '--' did not match any file(s) known to git.

  - an error when -- wasn't used, and the first argument is ambiguous
    (i.e. can be interpreted as both ref and as path).

Signed-off-by: Pierre Habouzit <madcoder@debian.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>