Cleanup gitignore to ignore all /bazel-* directories.

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Change-Id: Ib75c9a2b86586efc07ff16dc44caa2e4f4e420c3
Reviewed-on: https://cr.bazel.build/9067
PiperOrigin-RevId: 148631186
MOS_MIGRATED_REVID=148631186

.gitignore

@@ -1,16 +1,17 @@
+# Ignore backup files.
 *~
+# Ignore Vim swap files.
 .*.swp
+# Ignore files generated by IDEs.
 /.classpath
 /.factorypath
 /.idea/
 /.project
 /.settings
 /WORKSPACE.user.bzl
-/bazel-bazel
-/bazel-bin
-/bazel-genfiles
-/bazel-io_bazel
-/bazel-out
-/bazel-testlogs
 /bazel.iml
+# Ignore all bazel-* symlinks. There is no full list since this can change
+# based on the name of the directory bazel is cloned into.
+/bazel-*
+# Ignore outputs generated during Bazel bootstrapping.
 /output/