The function of this method is to defer the logic of generating a commit
message to template.go, and eventually remove that logic from
commands/pull_request.go

Also adds tests for this new method, which also modifies the
fixtures/test_repo.go file to accomidate a test repo with a `.github` PR
dir.

This allows the github.GeneratePRTemplate() function to autofill the
pull request title for a single commit pull request with either:

* The entire commit message, if it is a single line commit message
* The first line of the commit message, if it is a multi-line commit
  message

As far as the code is concerned, both cases above are interpreted in the
same fashion, but the expectations of the user are defined with each of
the new test cases in github/template_test.go

FYI, this commit message would be an example of a multi line commit
message :)

Implements the GeneratePRTemplate in pull_request.go instead of the
custom logic that previously existed in the file.  Addresses:

* Title being defaulted as the first line of the template
* Commit messages not defaulting as the title only properly

For the longest time I was assuming that this was a whitespace change
with `git diff`, even though I had fixed it when building this a few
weeks back, so I decided to not bother adding it to the previous
commits.  When I removed it and re-ran the tests, things stopped
working... oops :)

These typos were hiding failing tests (i.e., two mistakes were canelling
each other out and making the tests pass accidentally).

The tests *should* have been failing because the new addGitHubTemplates
function was writing GitHub templates to the wrong place in the test
repo fixture. They need to be written inside the repo, not in the same
directory as the repo.