oid: introduce `git_oid_raw_cpy`

Now that oids are type-aware, they use their type to understand how many
bytes to copy.  Some callers may need to copy the raw bytes of the
object id.

This is equivalent to a memcpy that is a little more semantic.

src/libgit2/oid.c

@@ -187,8 +187,7 @@ int git_oid_fromraw(git_oid *out, const unsigned char *raw)
 
 int git_oid_cpy(git_oid *out, const git_oid *src)
 {
-	memcpy(out->id, src->id, sizeof(out->id));
-	return 0;
+	return git_oid_raw_cpy(out->id, src->id);
 }
 
 int git_oid_cmp(const git_oid *a, const git_oid *b)

src/libgit2/oid.h

@@ -55,6 +55,14 @@ GIT_INLINE(int) git_oid_raw_cmp(
 	return memcmp(sha1, sha2, GIT_OID_RAWSZ);
 }
 
+GIT_INLINE(int) git_oid_raw_cpy(
+	unsigned char *dst,
+	const unsigned char *src)
+{
+	memcpy(dst, src, GIT_OID_RAWSZ);
+	return 0;
+}
+
 /*
  * Compare two oid structures.
  *