Honor the `commit.template` config variable

[ravicious]

Hi,

I discovered GitX-dev because GitX doesn't support the commit.template config var and I was looking for a lightweight client that does. Turns out neither GitX nor GitX-dev supports it. That's why I decided to create this feature request.

Expectation: If the commit.template var is set to some path, GitX-dev will use the contents of the file under the path to pre-fill the commit message field.

Currently setting the commit.template var doesn't affect GitX-dev behavior.