This patch adds some functions to mdiff.py that will be called from run-tests.py
which will be added in the second part of this patch series.
The future changes to run-tests.py will enable one to use mdiif
to diff rather than unidiff whenever the mercurial is installed in the system.
Why do I need to split the patches into two ?
The reason to split what would have been a single patch into two is because
for the next patch to be able to use the mdiff during tests,
this revision has to be there in the system's mercurial installation.