checkpatch: Merge continuation-header handling from .38

This commit is a cherry pick and squash of commits:
85c2ee62d37c19456c6dc83db262123956f010ac
4bc7c6001daba7d4037f54f67bae7fa90f759402

  checkpatch: Handle continuation headers.

  Continuation headers baffle checkpatch, as it can only operate
  on one line of context at a time.  When continuation headers are found,
  put them up with the header they're continuing so the whole thing
  can be parsed in a single line of context.

  checkpatch: Don't treat diffs as patches.

  The patch-header cleanup code assumed that it would only ever
  see patches, which was of course hogwash.  This lead to crazy
  results as it tried to wrap what it thought were continuation
  lines at the beginnings of raw diffs.

Change-Id: I2d22ed056f8203ae6dae473b01ae66bc1963ae75
Signed-off-by: Gregory Bean <gbean@codeaurora.org>
Signed-off-by: Rishabh Bhatnagar <rishabhb@codeaurora.org>