tests/test-diffdir

Avoid extra filelogs entries.

Right now, there are some situations in which localrepo.filecommit can
create filelog entries even though they're not needed.  For example:

- permissions for a file have changed;

- qrefresh can create a filelog entry identical to its parent (see the
  added test);

- convert-repo creates extra filelog entries in every merge where the
  first parent has added files (for example, changeset ebebe9577a1a of
  the kernel repo added extra filelog entries to files in the
  arch/blackfin directory, even though the merge should only touch the
  drivers/ata directory).  This makes "hg log file" in a converted repo
  less useful than it could be, since it may mention many merges that
  don't actually touch that specific file.

They all come from the same basic problem:  localrepo.commit (through
filecommit) creates new filelog entries for all files passed to it
(except for some cases during a merge).

Patch and test case provided by Benoit.

This should fix issue351.

#!/bin/sh

hg init
touch a
hg add a
hg ci -m "a" -d "1000000 0"

echo 123 > b
hg add b
hg diff --nodates

hg diff --nodates -r tip

echo foo > a
hg diff --nodates

hg diff -r ""
hg diff -r tip -r ""

true