diff-merges: let "-m" imply "-p"

diff-merges: let "-m" imply "-p"

Fix long standing inconsistency between -c/--cc that do imply -p on
one side, and -m that did not imply -p on the other side.

Change corresponding test accordingly, as "log -m" output should now
match one from "log -m -p", rather than from just "log".

Change documentation accordingly.

NOTES:

After this patch

  git log -m

produces diffs without need to provide -p as well, that improves both
consistency and usability. It gets even more useful if one sets
"log.diffMerges" configuration variable to "first-parent" to force -m
produce usual diff with respect to first parent only.

This patch, however, does not change behavior when specific diff
format is explicitly provided on the command-line, so that commands
like

  git log -m --raw
  git log -m --stat

are not affected, nor does it change commands where specific diff
format is active by default, such as:

  git diff-tree -m

It's also worth to be noticed that exact historical semantics of -m is
still provided by --diff-merges=separate.

Signed-off-by: Sergey Organov <sorganov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>