tests/test-diffdir.t

largefiles: restore normal 'clone -u' and 'clone -U' functionality

Previously, tip would be checked out regardless of the -u or -U parameter.  I'm
not sure what the 'required for successful walkchangerevs' comment meant, but it
appears to reference code which has since moved to downloadlfiles() in
7d6a660ca151.  Perhaps it was to force caching when the -U parameter is given?
The price of this change is that -U --all-largefiles won't cache anything.  That
will be fixed next.

Note that X + Y in the 'X largefiles updated, n removed' and 'Y additional
largefiles cached' lines do not add up to the same values in these tests, but
all of the largefiles have been downloaded.  The reason being that several
largefiles have the same content (eb7338044 is pointed to by sub/large2, large3
and sub/large4).  In the 'clone -u 1' operation, this largefile is cached to
populate the working directory, even without --all-largefiles.  That means the
file isn't downloaded again and cached in the rev where large3 and sub/large4
both point to this file.  Downloading that one file in that one rev seems to be
counted twice with 'clone -u 0'.  (Maybe it is also being downloaded twice?)

  $ 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]