Ignore Bazel directories in .gitignore

If I check out TensorFlow to a directory named tensorflow-clean
then Bazel is going to create a subdirectory named
bazel-tensorflow-clean. So we really need to have a wildcard
pattern.
Change: 136559940

.gitignore

@@ -1,10 +1,5 @@
 node_modules
-/bazel-bin
-/bazel-genfiles
-/bazel-out
-/bazel-tensorflow
-/bazel-testlogs
-/bazel-tf
+/bazel-*
 /third_party/py/numpy/numpy_include
 /tools/bazel.rc
 /util/python/python_include