Diff error on first revision using external diff tool

[Greg]

Using QBzr as part of TortoiseBZR I get the following error message when attempting to diff the 1st revision:

bzr: ERROR: bzrlib.errors.NoSuchRevision: KnitPackRepository('file:///C:/TestBazaar/.bzr/repository/') has no revision ('None',)
... File "bzrlib\builtins.pyo", line 1806, in run
  File "bzrlib\diff.pyo", line 338, in _get_trees_to_diff
...
  File "bzrlib\repository.pyo", line 2187, in _iter_inventory_xmls
NoSuchRevision: KnitPackRepository('file:///C:/TestBazaar/.bzr/repository/') has no revision ('None',)

bzr 1.15 on python 2.5.2 (win32)
arguments: ['C:\\Program Files\\Bazaar\\bzr.exe', 'qsubprocess', '"diff" "--using=C:/program files/winmerge/winmergeu.exe" -r revid:None..revid:REMOVED "src"']

Diffing other revisions works fine.
Diffing using the build in diff also works fine.

Steps to repro:
1) right click source file -> Tortoise Bazaar -> Log
2) right click Rev 1 -> Show file differences -> WinMerge (custom diff tool)
3) observe error box titled External Diff with contents pasted above.

[Robert Collins (lifeless)]

Fairly rare circumstance but well worth supporting.