tests/test-diffdir.t

run-tests: allow Test.run() to run multiple times

Test.run() can now be executed multiple times on the same Test instance.
This feature is currently unused and there are no plans to implement it.
The main reason for this work was to refactor testtmp, replacements, and
env to be run-time specific as opposed to Test instance specific.

  $ hg init
  $ touch a
  $ hg add a
  $ hg ci -m "a"

  $ echo 123 > b
  $ hg add b
  $ hg diff --nodates
  diff -r 3903775176ed b
  --- /dev/null
  +++ b/b
  @@ -0,0 +1,1 @@
  +123

  $ hg diff --nodates -r tip
  diff -r 3903775176ed b
  --- /dev/null
  +++ b/b
  @@ -0,0 +1,1 @@
  +123

  $ echo foo > a
  $ hg diff --nodates
  diff -r 3903775176ed a
  --- a/a
  +++ b/a
  @@ -0,0 +1,1 @@
  +foo
  diff -r 3903775176ed b
  --- /dev/null
  +++ b/b
  @@ -0,0 +1,1 @@
  +123

  $ hg diff -r ""
  hg: parse error: empty query
  [255]
  $ hg diff -r tip -r ""
  hg: parse error: empty query
  [255]