merge-recursive: introduce merge_options

merge-recursive: introduce merge_options

This makes it possible to avoid passing the labels of branches as
arguments to merge_recursive(), merge_trees() and
merge_recursive_generic().

It also takes care of subtree merge, output buffering, verbosity, and
rename limits - these were global variables till now in
merge-recursive.c.

A new function, named init_merge_options(), is introduced as well, it
clears the struct merge_info, then initializes with default values,
finally updates the default values based on the config and environment
variables.

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>