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.

One additional thing I did was when correlating "Show merge commits" and "Show revision graph", so that revision graph is never shown when merge commits are not shown. Reason being a) hiding merge commits breaks revision graph, b) bad performance hit.

![capture](https://f.cloud.github.com/assets/212700/608171/df2c1e64-cd56-11e2-9cad-f7c92dc88085.png)