[Dive] Simplifies the .gitignore file.

src/plugins/dive/.gitignore

-*~
-*.cm*
-*.annot
-*.o
-*.check_mli_exists
-*.generated
-autom4te.cache
-Makefile
-config.status
-config.log
-configure
-*_DEP
-local_config.ml
-.depend
-ptests_local_config.ml
-result
-ptests_config
+/Makefile
+/tests/*/result
+/tests/ptests_config