Fix fast forward merge behavior.
Enhance merge test coverage to better test possible outcomes.
Release LibGit2Sharp v0.16.0
Added an option to choose if commit message is prettified or not

Prettifing the message includes:
* Removing empty lines from the beginning and end.
* Removing trailing spaces from every line.
* Turning multiple consecutive empty lines between paragraphs into just one empty line.
* Ensuring the commit message ends with a newline.
* Removing every line starting with "#".

Note that this has nothing to do with the core.autocrlf option.