checkpatch: improve Gerrit Change-Id: test

The Gerrit Change-Id: entry is sometimes placed after a Signed-off-by:
line.  When this occurs, the Gerrit warning is not currently emitted as
the first Signed-off-by: signature sets a flag to stop looking.

Change the test to add a test for the --- patch separator and emit the
warning before any before the --- and also before any diff file name.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: John Stultz <john.stultz@linaro.org>
Link: http://lkml.kernel.org/r/2f6d5f8766fe7439a116c77ea8cc721a3f2d77a2.camel@perches.com


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