Benchmarking incremental builds

[zhengwei143]

Description of the problem / feature request:

Currently bazel-bench isn't able to benchmark incremental builds due to the following reasons:

1. Does not allow running --warm_up=n runs prior to the benchmark --runs=n
2. Does not allow toggling --shutdown and --clean to allow us to toggle whether or not to run blaze shutdown / clean (currently runs both by default).
3. Does not allow patching of code, should be done via specifying a --patch_file.

Feature requests: what underlying problem are you trying to solve with this feature?

Allows users to benchmark the incremental case, by:

1. Specifying --warm_up runs to warm up the blaze cache.
2. Do not shutdown or clean in between runs so that we run with a warm blaze cache.
3. Apply the patch file in-between runs to simulate code changes.