histedit: add experimental config for using the first word of the commit
This allows users to start a commit with "verb! ..." so that when this is
opened in histedit, the default action will be "verb". For example, "roll! foo"
will default to the action "roll". Currently, we'll allow any known verb to be
used but this is experimental.
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.