git log -p -m: document -m and honor --first-parent

git log -p -m: document -m and honor --first-parent

git log -p -m is used to show one merge entry per parent, with an
appropriate diff; this can be useful when examining histories where
full set of changes introduced by a merged branch is interesting, not
only the conflicts.

This patch properly documents the -m switch, which has so far been
mentioned only as a fairly special diff-tree flag.

It also makes the code show full patch entry only for the first parent
when --first-parent is used. Thus:

	git log -p -m --first-parent

will show the history from the "main branch perspective", while also
including full diff of changes introduced by other merged in branches.

Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>