merge, pull: introduce '--(no-)stat' option

merge, pull: introduce '--(no-)stat' option

This option has the same effect as '--(no-)summary' (i.e. whether to
show a diffsat at the end of the merge or not), and it is consistent
with the '--stat' option of other git commands.

Documentation, tests, and bash completion are updaed accordingly, and the
old --summary option is marked as being deprected.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>