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.