build with pristine-tar is unable to proceed with a dirty tree

[Nish Aravamudan]

We have put in code (c52db813e) to not build if the tree is dirty. This is overkill, so that pristine-tar is used, because `gbp` can't handle the tree being dirty (I believe it has to manipulate the working-tree). Either gbp should be adjusted to use a working-dir distinct from the current one, or we should dupe our repo somewhere else while we run some specific commands in it (and then put the results where we expect them to be)).

The latter solution is more generic and would also resolve LP: #1708203.

[Nish Aravamudan (nacc)]

This is now possible, so we can make this work, I think.

[Nish Aravamudan (nacc)]

Given that we, for now, are not going to allow building dirty trees, this is not as important of a problem to solve. It can be, in general, if we decide it should be, by using a temporary directory with a temporary worktree in a subdirectory, and extract the pristine-tar tarballs to the temporary directory, then copy them to the parent directory.

[Nish Aravamudan (nacc)]

@Robie, I'm assigning to you, for now, because it's a matter of questioning what the various methods in build.py should do when in a given repository state.