SubmittingPatches: document the use of git

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 <[email protected]>
Acked-by: Borislav Petkov <[email protected]>
Cc: Rob Landley <[email protected]>
Cc: Randy Dunlap <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>