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.
hg diff uses mdiff which which gives simpler and more readable diffs than unidiff
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.