grep: add --diff flag
Adds a diff flag, which works exactly same as all, in fact since
--all searches diffs, there diff is a better name for it.
The all flag is still here for backward compatibility reasons.
Some major tests for all has been picked and added for diff.
Differential Revision: https://phab.mercurial-scm.org/D3763
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.