hgview #5727418 upgrade old cmtable syntax to @command syntax [open]

I managed to get hgview to build (from sources) on OS X 10.11, but there's a warning message every time I run hg:

```
devel-warn: missing attribute 'norepo', use @command decorator to register 'hgview'
(compatibility will be dropped after Mercurial-3.8, update your code.) at: /usr/local/lib/python2.7/site-packages/hgext/mq.py:3570 (dotable)
```

This seems to be an outdated way to register commands. There is some documentation available at https://www.mercurial-scm.org/wiki/WritingExtensions, though it is not particularly clear.

FYI: To get hgview to install properly on OS X, I needed to modify `doc/Makefile` slightly, as the following diff shows.

```
diff -r fe00511ad0ad doc/Makefile
--- a/doc/Makefile Thu Nov 12 18:05:24 2015 +0100
+++ b/doc/Makefile Wed May 04 02:23:48 2016 +0100
@@ -13,7 +13,7 @@
html: $(HTML)

%: %.xml
- xmlto man $*.xml
+ xmlto --skip-validation man $*.xml

%.xml: %.txt
asciidoc -d manpage -b docbook -o $@ $<
```