reflog: move the reflog implementation into refdb_fs

References and their logs are logically coupled, let's make it so in
the code by moving the fs-based reflog implementation to live next to
the fs-based refs one.

As part of the change, make the function take names rather than
references, as only the names are relevant when looking up and
handling reflogs.

include/git2/sys/refdb_backend.h

@@ -119,6 +119,38 @@ struct git_refdb_backend {
 	 * provide this function; if it is not provided, nothing will be done.
 	 */
 	void (*free)(git_refdb_backend *backend);
+
+	/**
+	 * Read the reflog for the given reference name.
+	 */
+	int (*reflog_read)(git_reflog **out, git_refdb_backend *backend, const char *name);
+
+	/**
+	 * Write a reflog to disk.
+	 */
+	int (*reflog_write)(git_refdb_backend *backend, git_reflog *reflog);
+
+	/**
+	 * Append an entry to the given reflog
+	 */
+	int (*reflog_append)(git_refdb_backend *backend, git_reflog *reflog,
+			     const git_oid *new_oid, const git_signature *committer,
+			     const char *msg);
+
+	/**
+	 * Rename a reflog
+	 */
+	int (*reflog_rename)(git_refdb_backend *_backend, const char *old_name, const char *new_name);
+
+	/**
+	 * Drop an entry from the reflog
+	 */
+	int (*reflog_drop)(git_refdb_backend *_backend, git_reflog *reflog,
+			   size_t idx, int rewrite_previous_entry);
+	/**
+	 * Remove a reflog.
+	 */
+	int (*reflog_delete)(git_refdb_backend *backend, const char *name);
 };
 
 #define GIT_REFDB_BACKEND_VERSION 1

include/git2/sys/reflog.h

@@ -0,0 +1,21 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#ifndef INCLUDE_sys_git_reflog_h__
+#define INCLUDE_sys_git_reflog_h__
+
+#include "git2/common.h"
+#include "git2/types.h"
+#include "git2/oid.h"
+
+GIT_BEGIN_DECL
+
+GIT_EXTERN(git_reflog_entry *) git_reflog_entry__alloc(void);
+GIT_EXTERN(void) git_reflog_entry__free(git_reflog_entry *entry);
+
+GIT_END_DECL
+
+#endif