Support for cargo update while behind proxy

[rohitjoshi]

Does cargo support update when behind proxy?

I tried
export ALL_PROXY=http://localhost:8098

but Cargo update command hangs at

cargo update -v
    Updating registry `https://github.com/rust-lang/crates.io-index`

Looking at the source, it seem to be using curl which does support ALL_PROXY. Not sure rust module for curl has the support though.

[rohitjoshi]

Note: export ALL_PROXY=http://localhost:8098 works fine for brew upgrade

[alexcrichton]

Unfortunately this is because libgit2 does not support proxies just yet. As a result I believe that this is a dupe of #636, but thanks for the report!