Add a git clean command before updating local copy

Periodic runs of manage_project can leave local copies
of some projects in bad state, stopping next runs to
work due to git conflicts. Execute a git clean prior to
the update to ensure the process will work.

Change-Id: Iac0291f97a75b671147568f212df5e0b18cd6f06
Merge "Add a git clean command before updating local copy"