Add --force to 'git fast-import' command-line

Now that we support arbitrarily reseating branches, the branch can be
reset to older SVN revisions or derive from a different branch point.

The "only fast-forward" rule of git branch imports gets in our way.
Note that the application of the rule is fairly unpredictable since the
checking happens only at 'checkpoint's.

Let's trust our handling of SVN history and override the sanity-check.
Note that we don't lose any information since reseated branches are
snapshotted before reset.

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