refs: Allow MERGE_HEAD in normalize_name()

Do not return with EINVALIDREFNAME when trying to normalize MERGE_HEAD's
name.

src/refs.c

@@ -1692,8 +1692,9 @@ static int normalize_name(char *buffer_out, const char *name, int is_oid_ref)
 	}
 
 	/* Object id refname have to contain at least one slash, except
-	 * for HEAD in a detached state */
-	if (is_oid_ref && !contains_a_slash && strcmp(name, GIT_HEAD_FILE))
+	 * for HEAD in a detached state or MERGE_HEAD if we're in the
+	 * middle of a merge */
+	if (is_oid_ref && !contains_a_slash && (strcmp(name, GIT_HEAD_FILE) && strcmp(name, GIT_MERGE_HEAD_FILE)))
 				return GIT_EINVALIDREFNAME;
 
 	/* A refname can not end with ".lock" */

src/refs.h

@@ -17,6 +17,7 @@
 #define MAX_GITDIR_TREE_STRUCTURE_PATH_LENGTH 100
 
 #define GIT_HEAD_FILE "HEAD"
+#define GIT_MERGE_HEAD_FILE "MERGE_HEAD"
 #define GIT_REFS_HEADS_MASTER_FILE GIT_REFS_HEADS_DIR "master"
 
 struct git_reference {