SubmittingPatches: document the use of git

Most of the mechanical portions of SubmittingPatches exist to help patch
submitters replicate the output of git.  Mention this explicitly, both
as a reminder that git will help with this process, and as signposting
to let git users know what they can safely skip.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Acked-by: Borislav Petkov <bp@suse.de>
Cc: Rob Landley <rob@landley.net>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>