Don't clobber branch/testing done/bugs if changesets don't have them.
Don't clobber branch/testing done/bugs if changesets don't have them.

A few people are using perforce but don't have VMware's fancy changeset
descriptions, so their testing done and bugs fields were getting clobbered when
they updated the diff. This isn't really nice of us.

This change fixes update_obj_with_changenum to only clobber those fields if the
changeset has them present.

Fixes bug 777.




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