Produce better error reporting when absurd git wrapper fails on a patch

Produce better error reporting when absurd git wrapper fails on a patch

during .dsc import.  Apropos of #848391.

gbp swallows the error from the first run (without --whitespace=fix).
Previously only that first run would do anything useful; the second
would complain about --whitespace=fix.  So failures of dpkg-source
would always produce a useless error message.

Instead, tolerate (and ignore) the --whitespace=fix option which gbp
passes during the second run.  This means that the second run tries to
do the actual work.

This only does extra work if the first run failed, and since the first
and second runs are now equivalent, that happens when the second run
fails too.  But in return for that extra work, we get a report which
actually mentions something that unexpectedly went wrong.

(The alternative would be to have absurd/git try to stash and then
reproduce its previous error, or to make more fragile assumptions
about gbp's behaviour.)

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