i18n: mark merge "upstream" messages for translation

Mark the merge messages that were added in v1.7.5-rc1~17^2 (merge:
merge with the default upstream branch without argument) by Junio C
Hamano for translation.
Signed-off-by: NÆvar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/merge.c

@@ -962,16 +962,16 @@ static int setup_with_upstream(const char ***argv)
 	const char **args;
 
 	if (!branch)
-		die("No current branch.");
+		die(_("No current branch."));
 	if (!branch->remote)
-		die("No remote for the current branch.");
+		die(_("No remote for the current branch."));
 	if (!branch->merge_nr)
-		die("No default upstream defined for the current branch.");
+		die(_("No default upstream defined for the current branch."));
 
 	args = xcalloc(branch->merge_nr + 1, sizeof(char *));
 	for (i = 0; i < branch->merge_nr; i++) {
 		if (!branch->merge[i]->dst)
-			die("No remote tracking branch for %s from %s",
+			die(_("No remote tracking branch for %s from %s"),
 			    branch->merge[i]->src, branch->remote_name);
 		args[i] = branch->merge[i]->dst;
 	}