buildrpm: do not break in presence of custom user configs

For example, if "hg log" was defined as an alias:

  # /etc/mercurial/hgrc
  [alias]
  log = log --graph

the buildrpm script would be surprised by log messages formatted in
unexpected ways, and bail out.

This patch sets HGPLAIN, effectively resetting all the user configs,
including log output, to a common state, making the build more
predictable across all the possible environments.

contrib/buildrpm

@@ -11,6 +11,8 b''
 
 BUILD=1
 RPMBUILDDIR="$PWD/rpmbuild"
+export HGPLAIN=
+
 while [ "$1" ]; do
     case "$1" in
     --prepare )