Commit message and tag fixes

[greened]

This fixes two issues with zip-downstream-fork.py:

• Inlined umbrella commits that are actually upstream commits retain their umbrella commit message to distinguish them from their upstream counterparts

• When a subproject is used as an umbrella (e.g. llvm), update the tag map so that tags pointing to the upstream subproject commits will migrate to the zippped history

[greened]

Ping. It'd be nice to get this merged for folks.