beautifygraph: don't warn about busted terminal if HGPLAIN is set stable

beautifygraph: don't warn about busted terminal if HGPLAIN is set

This was breaking some automation for users that discovered the
extension and turned it on.

Differential Revision: https://phab.mercurial-scm.org/D4338

--- a/hgext/beautifygraph.py	Sun Aug 19 13:27:02 2018 +0900
+++ b/hgext/beautifygraph.py	Mon Aug 20 16:33:48 2018 -0400
@@ -77,6 +77,9 @@
     return orig(ui, graph, *args, **kwargs)
 
 def extsetup(ui):
+    if ui.plain('graph'):
+        return
+
     if encoding.encoding != 'UTF-8':
         ui.warn(_('beautifygraph: unsupported encoding, UTF-8 required\n'))
         return
@@ -86,8 +89,5 @@
                   'monospace narrow text required\n'))
         return
 
-    if ui.plain('graph'):
-        return
-
     extensions.wrapfunction(graphmod, 'outputgraph', outputprettygraph)
     extensions.wrapfunction(templatekw, 'getgraphnode', getprettygraphnode)