libgit2: Disable Security.framework transport on Darwin.

When used by cargo, `libgit2` crashes with a Segmentation Fault on
Darwin. This crash is somehow connected to the `Security.framework`
native to Darwin, and while being easy to reproduce, is very hard to
track down.

This commit introduces a patch to `libgit2` which disables the
troublesome transport and instead depends on `libcurl`. The patch also
adds support for `SSL_CERT_FILE` to `libgit2`.

Upstream tracking issue is
https://github.com/libgit2/libgit2/issues/3885.