git-svn: extra safety for noMetadata and useSvmProps users

git-svn: extra safety for noMetadata and useSvmProps users

Make sure we flush our userspace buffers and and fsync(2)
.rev_db information to disk if we use these options because
we really don't want to lose this information.

Also, disallow --use-svm-props and --no-metadata from the
command-line because history will be inconsistent if they're
only used occasionally.  If a user wants to use these options,
they must be set in the config so they're always on.

Signed-off-by: Eric Wong <normalperson@yhbt.net>