[Pkg-mono-devel] Working around "unrepresentable changes to source"

[Weeble]

If I do "apt-get source mono", apply some patches to the source tree
and then run dpkg-buildpackage, I can build a new version of the mono
package. But if I then try to apply another patch and repeat
dpkg-buildpackage, I get the error "dpkg-source: unrepresentable
changes to source". I understand this is because the mono build
process makes changes to the source tree during the build, is that
right? What I'd like to know is if there's any way to work around this
and do a fresh build without discarding the entire source tree,
running apt-get source mono and then applying the patches I need
again?