convert: add a mode where mercurial_sink skips empty revisions. The getchanges function of some converter_source classes can return some false positives. I.e. they sometimes claim that a file "foo" was changed in some revision, even though its contents are still the same. convert_svn is particularly bad, but I think this can also happen with convert_cvs and, at least in theory, with mercurial_source. For regular conversions this is not really a problem - as long as getfile returns the right contents, we'll get a converted revision with the right contents. But when we use --filemap, this could lead to superfluous revisions being converted. Instead of fixing every converter_source, I decided to change mercurial_sink to work around this problem. When --filemap is used, we're interested only in revisions that touch some specific files. If a revision doesn't change any of these files, then we're not interested in it (at least for revisions with a single parent; merges are special). For mercurial_sink, we abuse this property and rollback a commit if the manifest text hasn't changed. This avoids duplicating the logic from localrepo.filecommit to detect unchanged files.

Mercurial's documentation is currently kept in ASCIIDOC format, which
is a simple plain text format that's easy to read and edit. It's also
convertible to a variety of other formats including standard UNIX man
page format and HTML.

To do this, you'll need to install ASCIIDOC:

 http://www.methods.co.nz/asciidoc/

To generate the man page:

 asciidoc -d manpage -b docbook hg.1.txt
 xmlto man hg.1.xml

To display:

 groff -mandoc -Tascii hg.1 | more

To create the html page (without stylesheets):

 asciidoc -b html4 hg.1.txt

(older asciidoc may want html instead of html4 above)