verify: use some intermediate variables instead of a multi-liner
This is shorter and easier to read as the indentation remains the same.
Differential Revision: https://phab.mercurial-scm.org/D10818
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
[10]
No dirstate backups are left behind
$ ls .hg/dirstate* | sort
.hg/dirstate