tests: add a test demonstrating failure to clean up dirstate backups stable

tests: add a test demonstrating failure to clean up dirstate backups

Differential Revision: https://phab.mercurial-scm.org/D1200

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-dirstate-backup.t	Fri Oct 20 05:53:33 2017 -0700
@@ -0,0 +1,19 @@
+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)
+