gitignore cleanup

* ignore typical backup files created by common editors
* move general ignore rules like "*.o" or "depend.mk" to top level gitignore
* ignore executables and test directories in target dir gitignore
  as these are local and there's no general catch-all pattern for them

query_classifier/.gitignore

@@ -1,5 +0,0 @@
-*.o
-*.so
-*.so.*
-depend.mk
-

query_classifier/test/.gitignore

@@ -0,0 +1,2 @@
+# binaries generated here
+testmain