Add a --deep_execroot startup option that puts the execroot under $OUTPUT_BASE/execroot/<workspace name> instead of $OUTPUT_BASE/<workspace name>.

This makes it possible to run Bazel in directories whose name clashes with one of the pre-existing files in the output base. Note that this option is turned off by default for now.

--
MOS_MIGRATED_REVID=109791053

src/main/cpp/blaze_startup_options.h

diff --git a/src/main/cpp/blaze_startup_options.h b/src/main/cpp/blaze_startup_options.h
index ca52c27..d382ffa 100644
--- a/src/main/cpp/blaze_startup_options.h
+++ b/src/main/cpp/blaze_startup_options.h
@@ -117,6 +117,10 @@
   // output_base.
   string output_user_root;
 
+  // Whether to put the execroot at $OUTPUT_BASE/$WORKSPACE_NAME (if false) or
+  // $OUTPUT_BASE/execroot/$WORKSPACE_NAME (if true).
+  bool deep_execroot;
+
   // Block for the Blaze server lock. Otherwise,
   // quit with non-0 exit code if lock can't
   // be acquired immediately.