Docs: Bring SubmittingPatches more into the git era
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 <b2ce64d5587c02f12e367eac59751145a0660c51@lwn.net>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs