checkpatch: add warning for lines starting with a '#' in commit log

Commit log lines starting with '#' are dropped by git as comments.
Add a check to emit a warning for these lines.

Also add a --fix option to insert a space before the leading '#' in
such lines.

Link: https://lkml.kernel.org/r/20201202205740.127986-1-dwaipayanray1@gmail.com


Signed-off-by: Dwaipayan Ray <dwaipayanray1@gmail.com>
Suggested-by: Joe Perches <joe@perches.com>
Suggested-by: Peilin Ye <yepeilin.cs@gmail.com>
Tested-by: Peilin Ye <yepeilin.cs@gmail.com>
Acked-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>