tests/test-mq-qsave.t

tests: add test for "log -r wdir() -p" (issue4871)

The issue has been fixed by 93bcc73df8d5, "cmdutil.changeset_printer: pass
context into showpatch()". This patch adds test to prevent future regression.

  $ echo "[extensions]" >> $HGRCPATH
  $ echo "mq=" >> $HGRCPATH

  $ hg init

  $ echo 'base' > base
  $ hg ci -Ambase
  adding base

  $ hg qnew -mmqbase mqbase

  $ hg qsave
  $ hg qrestore 2
  restoring status: hg patches saved state