commit: --cleanup is a message option

commit: --cleanup is a message option

In the usage message for "git commit", the --cleanup option appeared
at the end, as one of the "contents options":

usage: git commit [options] [--] <filepattern>...
...
Commit message options
...
Commit contents options
...
    --allow-empty         ok to record an empty change
    --cleanup <default>   how to strip spaces and #comments from message

This is confusing, in part because it makes it ambiguous whether
--allow-empty, just above, refers to an empty diff or an empty message.

Move --cleanup into the 'message options' group.  Also add a pair of
comments to prevent similar oversights in the future.

Signed-off-by: Greg Price <price@ksplice.com>