Docs: Bring SubmittingPatches more into the git era

Much of the information in SubmittingPatches shows its pre-git history.
Clean that up a bit and rephrase things with the assumption that developers
will be using git.  Also rewrite the "pull requests" section and include
information on using signed tags.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>

diff --cc Documentation/SubmittingPatches
Simple merge