histedit: have dropmissing abort on empty plan
We noticed that many users have the intuition of laving the editor empty when
they want to abort the operation. The fact that dropmissing allows user to
delete all edited commits is not intuitive even for users that asked for it.
Let's prevent people from this footgun.
Mercurial's documentation is kept in reStructuredText format, which is
a simple plain text format that's easy to read and edit:
http://docutils.sourceforge.net/rst.html
It's also convertible to a variety of other formats including standard
UNIX man page format and HTML. You'll need to install Docutils:
http://docutils.sourceforge.net/
Use the Makefile in this directory to generate the man and HTML pages.