Reorganize `git-log' man page to clarify common diff options.

Reorganize `git-log' man page to clarify common diff options.

This will reduce considerably the common confusion where people miss the
`--follow' option, and wonder why `-M'/`-C' is not working.

* Move the diff options include to after the log-specific flags, and add
  a "Common diff options" subtitle before them.  (These options apply
  only when patches are shown, which is not a common use case among
  newbies, so having them first is confusing.)

* Move the `--follow' description to the top of the listed options.  The
  options before that seem less important: `--full-diff' applies only
  when patches are shown, `--source' and `--decorate' are less useful
  with many common commit specifications.

* Clarify that `--follow' works only for a single path argument.

Signed-off-by: Eli Barzilay <eli@barzilay.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>