Don't do non-fastforward updates in fast-import.

Don't do non-fastforward updates in fast-import.

If fast-import is being used to update an existing branch of
a repository, the user may not want to lose commits if another
process updates the same ref at the same time.  For example, the
user might be using fast-import to make just one or two commits
against a live branch.

We now perform a fast-forward check during the ref updating process.
If updating a branch would cause commits in that branch to be lost,
we skip over it and display the new SHA1 to standard error.

This new default behavior can be overridden with `--force`, like
git-push and git-fetch.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>