cmdutil: introduce 'editform' to distinguish the purpose of commit text editing
This information will be used to switch '[committemplate] changeset'
definition according to its purpose in the subsequent patch.
This information also makes it easier to hook commit text editing only
in the specific cases.
To run the tests, do:
cd tests/
python run-tests.py
See http://mercurial.selenic.com/wiki/WritingTests for
more information on writing tests.