hash: add a function to look up hash algo structs

In C, it's easy for us to look up a hash algorithm structure by its
offset by simply indexing the hash_algos array.  However, in Rust, we
sometimes need a pointer to pass to a C function, but we have our own
hash algorithm abstraction.

To get one from the other, let's provide a simple function that looks up
the C structure from the offset and expose it in Rust.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

src/hash.rs

@@ -12,6 +12,7 @@
 
 use std::error::Error;
 use std::fmt::{self, Debug, Display};
+use std::os::raw::c_void;
 
 pub const GIT_MAX_RAWSZ: usize = 32;
 
@@ -177,4 +178,17 @@ impl HashAlgorithm {
             HashAlgorithm::SHA256 => &Self::SHA256_NULL_OID,
         }
     }
+
+    /// A pointer to the C `struct git_hash_algo` for interoperability with C.
+    pub fn hash_algo_ptr(self) -> *const c_void {
+        unsafe { c::hash_algo_ptr_by_number(self as u32) }
+    }
+}
+
+pub mod c {
+    use std::os::raw::c_void;
+
+    extern "C" {
+        pub fn hash_algo_ptr_by_number(n: u32) -> *const c_void;
+    }
 }