Unwrap lines in requestpull default message

[zombiezen]

Git commit messages typically wrap at 80 columns or so. GitHub-flavored Markdown interprets such line wraps as a soft linebreak, which may be rendered as a space or a hard line break. Pull request messages interpret this as the latter, which yields funny rendering.

It would be nice if gg automatically wrapped such lines, but I commonly have a block of "Updates #XXX" or "Fixes #XXX" one-per-line, which should not be line-wrapped. That pattern may need to be special-cased if I can't come up with a more general rule for what shouldn't be unwrapped.