extdiff: register the configuration generated commands with a help category

Otherwise, 'extdiff' shows up under file management and the rest of the commands
are at the bottom under 'Uncategorized'.

hgext/extdiff.py

@@ -429,6 +429,7 b' def uisetup(ui):'
             if args:
                 cmdline += ' ' + args
         command(cmd, extdiffopts[:], _('hg %s [OPTION]... [FILE]...') % cmd,
+                helpcategory=command.CATEGORY_FILE_CONTENTS,
                 inferrepo=True)(savedcmd(path, cmdline))
 
 # tell hggettext to extract docstrings from these functions: