Basic reflog support

For now it simply includes reading in the reflog as a list of
entries. This does mean that the drop/append functions don't need to be
wrapped, as we can use erlang's list handling.

It however also means that we cannot write this back out, as there is
currently no way to create an empty reflog that we can fill with our own
list.

c_src/reflog.c

@@ -0,0 +1,66 @@
+#include "geef.h"
+#include "repository.h"
+#include "reference.h"
+#include "oid.h"
+#include "signature.h"
+#include <string.h>
+#include <git2.h>
+
+ERL_NIF_TERM
+geef_reflog_read(ErlNifEnv *env, int argc, const ERL_NIF_TERM argv[])
+{
+	git_reflog *reflog;
+	geef_repository *repo;
+	ErlNifBinary bin;
+	int error;
+	size_t count, i;
+	ERL_NIF_TERM list;
+
+	if (!enif_get_resource(env, argv[0], geef_repository_type, (void **) &repo))
+		return enif_make_badarg(env);
+
+	if (!enif_inspect_iolist_as_binary(env, argv[1], &bin))
+		return enif_make_badarg(env);
+
+	if (!geef_terminate_binary(&bin))
+		return geef_oom(env);
+
+	if ((error = git_reflog_read(&reflog, repo->repo, (char *)bin.data)) < 0)
+		return geef_error(env);
+
+	count = git_reflog_entrycount(reflog);
+	list = enif_make_list(env, 0);
+
+	for (i = count; i > 0; i--) {
+		ErlNifBinary id_old, id_new, message;
+		ERL_NIF_TERM committer, tentry;
+		const git_reflog_entry *entry;
+
+		entry = git_reflog_entry_byindex(reflog, i-1);
+
+		if (geef_oid_bin(&id_old, git_reflog_entry_id_old(entry)))
+			goto on_oom;
+
+		if (geef_oid_bin(&id_new, git_reflog_entry_id_new(entry)))
+			goto on_oom;
+
+		if (geef_signature_to_erl(&committer, env, git_reflog_entry_committer(entry)))
+			goto on_oom;
+
+		if (geef_string_to_bin(&message, git_reflog_entry_message(entry)))
+			goto on_oom;
+
+		tentry = enif_make_tuple5(env, atoms.reflog_entry, committer,
+					  enif_make_binary(env, &id_old),
+					  enif_make_binary(env, &id_new),
+					  enif_make_binary(env, &message));
+		list = enif_make_list_cell(env, tentry, list);
+	}
+
+	git_reflog_free(reflog);
+	return enif_make_tuple2(env, atoms.ok, list);
+
+on_oom:
+	git_reflog_free(reflog);
+	return geef_oom(env);
+}