41952: Use HGPLAIN instead of HGRCPATH in VCS_INFO_get_data_hg

Fixes prompt indicator in case if the repository requires a non-default
feature. With HGRCPATH, it results in:

abort: repository requires features unknown to this Mercurial: ...
(see https://mercurial-scm.org/wiki/MissingRequirement for more information)

Which, in turn, results in "default" getting reported regardless of what
is the current state of the repo.

ChangeLog

@@ -1,3 +1,8 @@
+2017-10-25  Alexey Sokolov  <sokolov@google.com>
+
+	* 41952: Functions/VCS_Info/Backends/VCS_INFO_get_data_hg: Use
+	HGPLAIN instead of HGRCPATH in VCS_INFO_get_data_hg
+
 2017-10-20  Christian Brabandt  <cb@256bit.org>
 
 	* 41943: Completion/Unix/Command/_vim: Completion: add --clean for

Functions/VCS_Info/Backends/VCS_INFO_get_data_hg

@@ -55,8 +55,8 @@ if zstyle -t ":vcs_info:${vcs}:${usercontext}:${rrn}" get-revision ; then
         zstyle -t ":vcs_info:${vcs}:${usercontext}:${rrn}" \
             "check-for-changes" || hgid_args+=( -r. )
 
-        local HGRCPATH
-        HGRCPATH="/dev/null" ${vcs_comm[cmd]} ${(z)hgid_args} 2> /dev/null \
+        local HGPLAIN
+        HGPLAIN=1 ${vcs_comm[cmd]} ${(z)hgid_args} 2> /dev/null \
             | read -r r_csetid r_lrev r_branch
     fi
 fi