upgrade: walk the source store file only once
I don't expect this to have a significant performance impact, but it seems
simpler and saner to do the operation only once and to keep the result around.
Set up
$ hg init repo
$ cd repo
Try to import an empty patch
$ hg import --no-commit - <<EOF
> EOF
applying patch from stdin
abort: stdin: no diffs found
[255]
No dirstate backups are left behind
$ ls .hg/dirstate* | sort
.hg/dirstate