libgit: add higher-level libgit crate

The C functions exported by libgit-sys do not provide an idiomatic Rust
interface. To make it easier to use these functions via Rust, add a
higher-level "libgit" crate, that wraps the lower-level configset API
with an interface that is more Rust-y.

This combination of $X and $X-sys crates is a common pattern for FFI in
Rust, as documented in "The Cargo Book" [1].

[1] https://doc.rust-lang.org/cargo/reference/build-scripts.html#-sys-packages

Co-authored-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Calvin Wan <calvinwan@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/libgit-sys/src/lib.rs

@@ -1,3 +1,5 @@
+use std::ffi::c_void;
+
 #[cfg(has_std__ffi__c_char)]
 use std::ffi::{c_char, c_int};
 
@@ -19,6 +21,8 @@ pub struct libgit_config_set {
 }
 
 extern "C" {
+    pub fn free(ptr: *mut c_void);
+
     pub fn libgit_user_agent() -> *const c_char;
     pub fn libgit_user_agent_sanitized() -> *const c_char;