Adding HGPLAIN to tramp's default environment

["Jordi Gutiérrez Hermoso" <jordigh@octave.org>]

Tramp was freezing for me when checking vc-registered on my remote hg
repos. Turns out my remote hg config was the problem, probably the
pager. As suggested by the hg docs,

$ hg help env.hgplain
    HGPLAIN
        When set, this disables any configuration settings that might change
        Mercurial's default output. This includes encoding, defaults, verbose
        mode, debug mode, quiet mode, tracebacks, and localization. This can
        be useful when scripting against Mercurial in the face of existing
        user configuration.

        Equivalent options set via command line flags or environment variables
        are not overridden.

Hence the following patch.

- Jordi G. H.

diff --git a/lisp/net/tramp-sh.el b/lisp/net/tramp-sh.el
--- a/lisp/net/tramp-sh.el
+++ b/lisp/net/tramp-sh.el
@@ -458,7 +458,7 @@ as given in your `~/.profile'."
     "EMACS=t" ;; Deprecated.
     ,(format "INSIDE_EMACS='%s,tramp:%s'" emacs-version tramp-version)
     "CDPATH=" "HISTORY=" "MAIL=" "MAILCHECK=" "MAILPATH=" "PAGER=\"\""
-    "autocorrect=" "correct=")
+    "autocorrect=" "correct=" "HGPLAIN=1")
   "List of environment variables to be set on the remote host.
 
 Each element should be a string of the form ENVVARNAME=VALUE.  An