Fix issue with filter not applied correctly during task _tags command

Also includes:

* Add cmake build directory + Session.vim files to .gitignore list

.gitignore

@@ -8,7 +8,9 @@ src/commands/libcommands.a
 src/columns/libcolumns.a
 *~
 .*.swp
+Session.vim
 package-config/osx/binary/task
+/build/
 CMakeFiles
 CMakeCache.txt
 cmake_install.cmake

src/commands/CmdTags.cpp

@@ -166,7 +166,7 @@ int CmdCompletionTags::execute (std::string& output)
   // Apply filter.
   Filter filter;
   std::vector <Task> filtered;
-  filter.subset (filtered);
+  filter.subset (tasks, filtered);
 
   // Scan all the tasks for their tags, building a map using tag
   // names as keys.