Terminology: Change "rewind" to "rewrite" where appropriate

Terminology: Change "rewind" to "rewrite" where appropriate

In #928473, Colin Watson writes:
>   the use of "rewind" as a synonym for "non-fast-forwarding", while
> somewhat common in git terminology, is unfortunate.  The terms seem
> to be borrowed from video playback systems, where "rewind" is often
> just the exact opposite of "fast-forward", and so when I see
> "rewinding history" in a few places in dgit(1) my initial
> interpretation is that it must mean "updating a ref to point to an
> ancestor of the commit that it previously pointed to", whereas I
> think dgit(1) means "any push that isn't a fast-forward".  I don't
> know if I'm the only one for whom it has that connotation.

This makes sense.  So, I am changing uses of "rewind" which do not
mean precisely going back to an ancestor.

I think we can often use the word "rewrite" for the more general
case, but there are some places where another wording is better.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>