Clarify new PRs’ sources and targets

In the text with which `git hub pull-request` pre-populates the pull
request message, clarify which repository and branch is the source and
which is the target.

Previously, the following (e.g.) was shown:

    > New GitHub Pull Request for
    >   repo: git@github.com:ingydotnet/git-hub
    >   branch: master

When I first saw this, I didn’t know whether this referred to the source
or target (and thus, whether I had given the correct arguments) until I
read the script.

Now, the following (e.g.) should be shown:

    > New GitHub Pull Request
    >
    > Requesting that...
    >   repo: git@github.com:8573/git-hub.git
    >   branch: 8573/clarify-new-pr-source-and-target/1
    > ...be pulled into...
    >   repo: git@github.com:ingydotnet/git-hub
    >   branch: master

Note: One may notice that the source repository resource-locator ends
with `.git` and the target repository resource-locator doesn’t. This is
just because that’s what I set as the resource-locator of my `fork`
remote; as far as GitHub cares, the `.git` extension doesn’t matter.