tests: add a test demonstrating failure to clean up dirstate backups
Differential Revision: https://phab.mercurial-scm.org/D1200
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]
A dirstate backup is left behind
$ ls .hg/dirstate* | sort
.hg/dirstate
.hg/dirstate.backup.import.* (glob)