lua: allow changing the displayed file of a window

Change the file displayed in a window by writing the new file name
to the window's file member.

vis-lua.c

@@ -2058,6 +2058,9 @@ static int window_newindex(lua_State *L) {
 			}
 			lua_pop(L, 1);
 			return ret;
+		} else if (strcmp(key, "file") == 0 && lua_isstring(L, 3)) {
+			const char* filename = lua_tostring(L, 3);
+			vis_window_change_file(win, filename);
 		}
 	}
 

vis.c

@@ -505,6 +505,17 @@ bool vis_window_reload(Win *win) {
 	return true;
 }
 
+bool vis_window_change_file(Win *win, const char* filename) {
+	File *file = file_new(win->vis, filename);
+	if (!file)
+		return false;
+	file->refcount++;
+	if (win->file)
+		file_free(win->vis, win->file);
+	win->file = file;
+	view_reload(win->view, file->text);
+}
+
 bool vis_window_split(Win *original) {
 	vis_doupdates(original->vis, false);
 	Win *win = window_new_file(original->vis, original->file, UI_OPTION_STATUSBAR);

vis.h

@@ -212,6 +212,8 @@ bool vis_window_new(Vis*, const char *filename);
 bool vis_window_new_fd(Vis*, int fd);
 /** Reload the file currently displayed in the window from disk. */
 bool vis_window_reload(Win*);
+/** Change the file currently displayed in the window. */
+bool vis_window_change_file(Win*, const char *filename);
 /** Check whether closing the window would loose unsaved changes. */
 bool vis_window_closable(Win*);
 /** Close window, redraw user interface. */