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
    the current state of the repo is.

diff --git a/Functions/VCS_Info/Backends/VCS_INFO_get_data_hg
b/Functions/VCS_Info/Backends/VCS_INFO_get_data_hg
index d4030125c..cd5ef321d 100644
--- a/Functions/VCS_Info/Backends/VCS_INFO_get_data_hg
+++ b/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