Steps to reproduce:

komar@thinkpad /home/komar % mkdir test
komar@thinkpad /home/komar % cd test 
komar@thinkpad /home/komar/test % darcs init 
komar@thinkpad /home/komar/test % echo 'hello world' > file
komar@thinkpad /home/komar/test % darcs add file 
komar@thinkpad /home/komar/test % darcs record -a
What is the patch name? patch 1
Do you want to add a long comment? [yn]n
Finished recording patch 'patch 1'
komar@thinkpad /home/komar/test % darcs mv file new_file
komar@thinkpad /home/komar/test % darcs record -a
What is the patch name? file -> new_file
Do you want to add a long comment? [yn]n
Finished recording patch 'file -> new_file'
komar@thinkpad /home/komar/test % darcs changes --xml-output
<changelog>                                                 
<patch author='Alexander Markov &lt;apsheronets@gmail.com&gt;'
date='20131219070126' local_date='Thu Dec 19 11:01:26 MSK 2013'
inverted='False'
hash='20131219070126-c9899-cb2d0a57d019babcce5ded57354cbcdeeccc015e'>
  <name>file -&gt; new_file</name>
  <comment>Ignore-this: ebc35b20aac43be11977847fd969b9b0</comment>
</patch>
<patch author='Alexander Markov &lt;apsheronets@gmail.com&gt;'
date='20131219070114' local_date='Thu Dec 19 11:01:14 MSK 2013'
inverted='False'
hash='20131219070114-c9899-389fbd5496ae4d849c16a2c499852e15b7b75669'>
  <name>patch 1</name>
  <comment>Ignore-this: 7bc120fdbbb64e5f13ddff2c80828765</comment>
</patch>
</changelog>
komar@thinkpad /home/komar/test % darcs diff new_file '--match=hash
20131219070114-c9899-389fbd5496ae4d849c16a2c499852e15b7b75669'         
                                                       
diff: old-test/new_file: No such file or directory
diff: new-test/new_file: No such file or directory
Thu Dec 19 11:01:14 MSK 2013  Alexander Markov <apsheronets@gmail.com>
  * patch 1

This is a reason why my website polling clients with error 500:
http://komar.bitcheese.net/ru/генерация-биткоинов?hash=20110714192936-c9899-033eb42cc44604c06613f2c7fae539e1e5c5c55a
but these links (to full patches, not changes to one page) are working:
http://komar.bitcheese.net/?hash=20110714192936-c9899-033eb42cc44604c06613f2c7fae539e1e5c5c55a

Please fix that.

works for me (test script added in patch2080)