Minor touch-ups to GitHub PULL_REQUEST_TEMPLATE
Previously, `PULL_REQUEST_TEMPLATE.md` included spaces before each item in the [GitHub task list](https://github.com/blog/1375-task-lists-in-gfm-issues-pulls-comments), which caused the tasks to become nested upon GUI reordering.

I made minor changes to other aspects of `PULL_REQUEST_TEMPLATE.md`. I removed the `-u` arg from the `git diff` command since its default. In `contributing.rst`, I switched `-u` to the long form of `--patch`, since it's later juxtaposed to `--name-only`.