Auto merge of #5069 - matklad:win7-y-u-no-tls, r=alexcrichton

Warn Windows 7 users about old TLS

An eyepatch for https://github.com/rust-lang/cargo/issues/5066.

@retep998 what would the best way to check for windows 7 specifically? Currently I use just `#[cfg(windows)]`, which might give false positives on 8 and 10.

diff --cc src/cargo/sources/git/utils.rs
Simple merge