highlight: eagerly discover plugin lexers while demandimport is off

Since highlight is only relevant for servers, it seems worthwhile to
just trigger this eagerly, which avoids really weird traceback
problems caused by demandimport messing with some of the lexer plugins.

Differential Revision: https://phab.mercurial-scm.org/D1619

hgext/highlight/highlight.py

@@ -22,8 +22,12 b' with demandimport.deactivated():'
     import pygments
     import pygments.formatters
     import pygments.lexers
+    import pygments.plugin
     import pygments.util
 
+    for unused in pygments.plugin.find_plugin_lexers():
+        pass
+
 highlight = pygments.highlight
 ClassNotFound = pygments.util.ClassNotFound
 guess_lexer = pygments.lexers.guess_lexer