Fix BookMergingTests
Fix BookMergingTests

Two tests were failing on my machine because Mercurial read the ~/.hgrc
file and couldn't load some enable extensions. This change sets the
HGRCPATH environment variable to the empty path which prevents loading
the ~/.hgrc file.

StephenMcConnel/BloomDesktop,andrew-polk/BloomDesktop,StephenMcConnel/BloomDesktop,andrew-polk/BloomDesktop,andrew-polk/BloomDesktop,JohnThomson/BloomDesktop,StephenMcConnel/BloomDesktop,andrew-polk/BloomDesktop,BloomBooks/BloomDesktop,JohnThomson/BloomDesktop,gmartin7/myBloomFork,BloomBooks/BloomDesktop,JohnThomson/BloomDesktop,JohnThomson/BloomDesktop,gmartin7/myBloomFork,gmartin7/myBloomFork,BloomBooks/BloomDesktop,gmartin7/myBloomFork,BloomBooks/BloomDesktop,StephenMcConnel/BloomDesktop,BloomBooks/BloomDesktop,BloomBooks/BloomDesktop,gmartin7/myBloomFork,andrew-polk/BloomDesktop,gmartin7/myBloomFork,StephenMcConnel/BloomDesktop,andrew-polk/BloomDesktop,StephenMcConnel/BloomDesktop,JohnThomson/BloomDesktop