Handle merge navigation in main view

Handle merge ops in main view and emit a message when they are
encountered outside.

As per review: jonas#525 (comment)

src/main.c

@@ -24,6 +24,7 @@
 #include "tig/stage.h"
 #include "tig/main.h"
 #include "tig/diff.h"
+#include "tig/search.h"
 
 /*
  * Main view backend
@@ -536,6 +537,11 @@ main_request(struct view *view, enum request request, struct line *line)
 		goto_id(view, "%(commit)^", true, false);
 		break;
 
+	case REQ_MOVE_NEXT_MERGE:
+	case REQ_MOVE_PREV_MERGE:
+		find_merge(view, request);
+		break;
+
 	default:
 		return request;
 	}

src/tig.c

@@ -294,7 +294,7 @@ view_driver(struct view *view, enum request request)
 
 	case REQ_MOVE_NEXT_MERGE:
 	case REQ_MOVE_PREV_MERGE:
-		find_merge(view, request);
+		report("Moving between merge commits is not supported by the %s view", view->name);
 		break;
 
 	case REQ_STOP_LOADING: