upgrade: don't perform anything if nothing to do
Before this patch, upgrade will process everything, re-clone all revlogs, write
requirements file again even there is no change to be made.
This patch makes it exit earlier.
Differential Revision: https://phab.mercurial-scm.org/D9695
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.