[PATCH V3] rebase: fail-fast the pull if working dir is not clean (BC)

[timeless]

Gregory Szorc wrote:
> Your reasoning about users wanting extra context to help them debug is sound.
> But I'm surprised nobody mentioned that the "error.Abort" exception takes an
> optional keyword "hint" argument that prints an extra message to provide that context.
> When we know why an abort occurred and/or what actions can be taken to prevent it,
> the "hint" message should provide that context.

+1