Fixing the git binary detection

Hi,

I had a strange issue with gollum. It was trying to access some folder named git I have. I found that it was grit, trying to use it as the git command.

I ended up looking at grit's code and found the problem.
It was testing that the tested path existed but not if it was an executable file.

Is pull request is a fix addressing that specific issue. I added tests too (they're not very pretty, sorry).

Just a question, is it for compatibility reasons that you're not using %x(which git) ?

Have a good day !