log: enable "-m" automatically with "--first-parent"

When using "--first-parent" to consider history as a single line of commits, git-log still defaults to treating merges specially, even though they could be considered as single commits in the linearized history (that just introduce all of the changes from the second and higher parents). Let's instead have "--first-parent" imply "-m", which makes something like: git log --first-parent -p do what you'd expect. Likewise: git log --first-parent -Sfoo will find "foo" in merge commits. No new test is needed; we'll tweak the output of the existing "--first-parent -p" test, which now matches the "-m --first-parent -p" test. The unchanged existing test for "--no-diff-merges" confirms that the user can get the old behavior if they want. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>