Auto merge of #11162 - pacak:master, r=ehuss

Expose libgit2-sys/vendored feature as vendored-libgit2

### What does this PR try to resolve?

Compiling cargo on MacOS can be problematic due present libraries, `cargo` already supports vendoring for openssl (and it helps pacak/cargo-show-asm#51 to some users). This pull request attempts to extend vendoring support to libgit2 in order to fix more foreign library issues: pacak/cargo-show-asm#50

While it is possible for me to enable it on my side (pacak/cargo-show-asm#52) a special care needs to keep versions in sync. Exposing this feature in `cargo` directly will make such fixes for my and other crates more robust.

### How should we test and review this PR?
Disabled by default means existing behavior is unchanged, testing with it enabled to confirm if it still works should do the trick.

Author: bors

Cargo.toml

@@ -115,4 +115,5 @@ doc = false
 [features]
 deny-warnings = []
 vendored-openssl = ["openssl/vendored"]
+vendored-libgit2 = ["libgit2-sys/vendored"]
 pretty-env-logger = ["pretty_env_logger"]