Various modifications

The first commit adds `pcache` to the Package-Requires, so the package manager can install it as a dependency when installing magit-gh-pulls.

Second commit introduces guessing the repo from the remote origin line in `.git/config`. This is much more convenient than adding a custom option for gh-pulls.

Third commit adds an option to merge the pull request into the branch it is intended for. It is convenient when you are sure you want to merge, since it automatically removes the branches it create to do the merge.

The last one removes the "Pull Request" section from Magit status if there are no pull requests, or the repo was not figured out. Before magit-gh-pulls just failed with errors if repo couldn't be deducted.