revset: add desc(string) to search in commit messages
Like keyword(), but does not search in filenames and users.
No grepdesc() or descgrep() added, because it might be bad to introduce
grepfoo() versions of too many string searches.
To run the tests, do:
cd tests/
python run-tests.py
See http://mercurial.selenic.com/wiki/WritingTests for
more information on writing tests.