git-debrebase: Improve error message for convert-from-gbp upstream discrepancy

git-debrebase: Improve error message for convert-from-gbp upstream discrepancy

Closes:#905078.

I don't intend to print `small' diffs because I think that kind of
dwim is probably more annoying than helpful.

I don't intend to factor this kind of pattern out throughout because:
The messages for these commands are generally not the actual diff rune
dgit or gdr ran, for various reasons (notably that dgit/gdr often use
object hashes).  The result is that there is little duplication.

So I think the situation now is OK.

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