cargo upgrade doesn't seem to see the latest crate

[ia0]

I believe this is the reason behind #875. Here is a repro case:

git clone git@github.com:ia0/data-encoding.git
cd data-encoding/lib/macro/internal
cargo upgrade -p syn -i --verbose --verbose

Note how the latest syn crate is 1.0.109 although it should be 2.0.58 at the time of this writing:

name          old req   compatible latest  new req   note    
====          =======   ========== ======  =======   ====    
data-encoding 2.6.0-git -          -       2.6.0-git excluded
syn           1         1.0.109    1.0.109 1                 

Now let's repro #875 to really see that the latest version is wrong:

sed -i 's/"1"/"2"/' Cargo.toml
cargo upgrade -p syn -i --verbose --verbose

We actually get a downgrade from 2 to 1:

name          old req   compatible latest  new req   note    
====          =======   ========== ======  =======   ====    
data-encoding 2.6.0-git -          -       2.6.0-git excluded
syn           2         -          1.0.109 1                 

Note that this is not because the requirement is only the major version. We can also use a full version and get the same behavior:

sed -i 's/"1"/"2.0.58"/' Cargo.toml

We still get a downgrade with the same wrong latest version:

name          old req   compatible latest  new req   note    
====          =======   ========== ======  =======   ====    
data-encoding 2.6.0-git -          -       2.6.0-git excluded
syn           2.0.58    -          1.0.109 1.0.109