Use git-merge in git-pull (second try).

Use git-merge in git-pull (second try).

This again makes git-pull to use git-merge, so that different merge
strategy can be specified from the command line.  Without explicit
strategy parameter, it defaults to git-merge-resolve if only one
remote is pulled, and git-merge-octopus otherwise, to keep the
default behaviour of the command the same as the original.

Also this brings another usability measure: -n flag from the command
line, if given, is passed to git-merge to prevent it from running the
diffstat at the end of the merge.

Signed-off-by: Junio C Hamano <junkio@cox.net>