checkpatch: add ability to coalesce commit descriptions on multiple lines

If a git commit description is split on consecutive lines, coalesce it
before testing.

This allows:

  commit <foo> ("some long
  description")

Signed-off-by: Joe Perches <joe@perches.com>
Reported-by: Paul Bolle <pebolle@tiscali.nl>
Tested-by: Paul Bolle <pebolle@tiscali.nl>

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>