[#5685] Converted commit log and last_commit_ids for git back to use SCM library

After some more empircal testing, it appears that my earlier
determination that calling out to git to determine the LCD info was too
expensive was due to the previous implementation of commit log not using
a generator and thus walking the entire history every time it was being
used instead of just until the changed paths were identified.

TimerMiddleware metrics for HEAD~ implementation:

    Total Time (ms)    Calls        Method
       48144              1    repo.LastCommit._build

TimerMiddleware metrics for this implementation:

    Total Time (ms)    Calls        Method
        1646              1    repo.LastCommit._build

This really needs to be tested against many repeated calls to
LastCommit._build, since the overhead of repeated calls to the external
git process might add up faster than the (significant) savings.

Signed-off-by: Cory Johns <johnsca@geek.net>