./python ./Tools/scripts/patchcheck.py
Getting the list of files that have been added/changed ... 0 files
Fixing whitespace ... 0 files
Fixing C file whitespace ... 0 files
Fixing docs whitespace ... 0 files
Docs modified ... NO
Misc/ACKS updated ... NO
Misc/NEWS updated ... NO
configure regenerated ... not needed
pyconfig.h.in regenerated ... not needed
robertc@lifeless-z140:~/work/cpython.hg$ hg diff
robertc@lifeless-z140:~/work/cpython.hg$ hg push ssh://hg@hg.python.org/cpython
pushing to ssh://hg@hg.python.org/cpython
searching for changes
remote: adding changesets
remote: adding manifests
remote: adding file changes
remote: added 1 changesets with 3 changes to 3 files
remote:  - file Lib/distutils/tests/test_core.py is not whitespace-normalized in ef5a2ba9df62
remote: * Run Tools/scripts/reindent.py on .py files or Tools/scripts/reindent-rst.py on .rst files listed above
remote: * and commit that change before pushing to this repo.
remote: transaction abort!
remote: rollback completed
remote: abort: pretxnchangegroup.checkwhitespace hook failed

The mistake I made was committing then running patchcheck.

I think it would make sense, if there are no changed files, to either 
a) error - it doesn't make sense to run like this, or
b) use the list of files from the most recent commit as the files to check.

I think b) would be easier on developers.