pull: pass git-merge's options to git-merge

pull: pass git-merge's options to git-merge

Specify git-merge's options in the option list, and pass any specified
options to git-merge.

These options are:

* -n, --stat, --summary: since d8abe14 (merge, pull: introduce
  '--(no-)stat' option, 2008-04-06)

* --log: since efb779f (merge, pull: add '--(no-)log' command line
  option, 2008-04-06)

* --squash: since 7d0c688 (git-merge --squash, 2006-06-23)

* --commit: since 5072a32 (Teach git-pull about --[no-]ff, --no-squash
  and --commit, 2007-10-29)

* --edit: since 8580830 ("git pull" doesn't know "--edit", 2012-02-11)

* --ff, --ff-only: since 5072a32 (Teach git-pull about --[no-]ff,
  --no-squash and --commit, 2007-10-29)

* --verify-signatures: since efed002 (merge/pull: verify GPG signatures
  of commits being merged, 2013-03-31)

* -s, --strategy: since 60fb5b2 (Use git-merge in git-pull (second
  try)., 2005-09-25)

* -X, --strategy-option: since ee2c795 (Teach git-pull to pass
  -X<option> to git-merge, 2009-11-25)

* -S, --gpg-sign: since ea230d8 (pull: add the --gpg-sign option.,
  2014-02-10)

Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>