Add Show merge commits option in commit view

Unchecking show merge commits adds --no-merges flag to the "git log" command.
This is quite useful when more than a couple developers actively working on a code base, causing large amounts of merge commits.

![capture](https://f.cloud.github.com/assets/212700/608171/df2c1e64-cd56-11e2-9cad-f7c92dc88085.png)