_set_description() was previously busted because of a variable name

mismatch.
_set_description() was previously making 2 hg process calls. This has

been optimized into a single process call.
The --follow argument has been removed because it is deprecated in

modern Mercurial clients and it only applies to `hg log <file>`, which

we weren't doing.
We also changed parsing of description messages so we can robustly

extract what description corresponds to which commit. This allows us to

easily strip off the undesired base commit.
Tests for summary and description guessing were added.
The behavior for description population is arguably wrong since the

first line of the tip summary is duplicated between the summary and

description. This could be changed with a follow-up.
Also, the previous behavior ordered descriptions from tip to base. This

patch reorders them from base to tip. It is the opinion of the patch

author that the description should be in commit order.

Tests added. Ran test suite locally.