Optimize Mercurial summary and description population; change behavior
The main goal of this patch is to change the --guess-description

behavior of the Mercurial client to not put a redundant summary line

in the guessed description.
To accomplish this, the functions for summary and description guessing

were merged. This means we only invoke `hg` once to get both the summary

and description. This is a slight performance win.
A new test to cover the new behavior is introduced. Existing test cases

have been adapted to contain multi-line commit messages so the code

is adequately tested.

Ran tests locally. I don't have hg-subversion installed, so I didn't run those tests.