oid: provide an oid type to hash type map

We intentionally separate oid types from hash types; a hash is a generic
hunk of bytes, an object id has meaning and backs an object on disk.  As
a result of this separation, we need a 1:1 mapping.

src/libgit2/oid.h

@@ -10,6 +10,7 @@
 #include "common.h"
 
 #include "git2/oid.h"
+#include "hash.h"
 
 #define GIT_OID_NONE { 0, { 0 } }
 
@@ -40,6 +41,18 @@ GIT_INLINE(size_t) git_oid_hexsize(git_oid_t type)
 	return 0;
 }
 
+GIT_INLINE(git_hash_algorithm_t) git_oid_algorithm(git_oid_t type)
+{
+	switch (type) {
+	case GIT_OID_SHA1:
+		return GIT_HASH_ALGORITHM_SHA1;
+	case GIT_OID_SHA256:
+		return GIT_HASH_ALGORITHM_SHA256;
+	}
+
+	return 0;
+}
+
 /**
  * Format a git_oid into a newly allocated c-string.
  *