log-tree: show_log: make commentary block delimiting reusable

log-tree: show_log: make commentary block delimiting reusable

In patches generated by git-format-patch, the area below the "---" line
following the commit message and before the actual 'diff' can be used
for commentary which the patch author wants to convey to readers of the
patch itself but not include in the commit message proper.

By default, the commentary area is empty, however, the --notes option
causes it to be populated with notes associated with the commit. In the
future, other options may be added which also insert content into the
commentary section.

To accommodate this, factor out the logic which delimits commentary
blocks from the commit message so that it can be re-used for upcoming
optional inserted content.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>