Add option to correct gitdir when syncing

In some cases, a user may wish to continue with a sync even though
it would require overwriting an existing git directory. This behavior
is not safe as a default because it could result in the loss of some
user data, but as an optional flag it allows the user more flexibility.

To support this, add a --force-sync flag to the sync command that will
attempt to overwrite the existing git dir if it is specified and the
existing git dir points to the wrong obj dir.

Change-Id: Ieddda8ad54e264a1eb4a9d54881dd6ebc8a03833