builtin-merge: avoid run_command_v_opt() for recursive and subtree

builtin-merge: avoid run_command_v_opt() for recursive and subtree

The try_merge_strategy() function always ran the strategy in a separate
process, though this is not always necessary. The recursive and subtree
strategy can be called without a fork(). This patch adds a check, and
calls recursive in the same process without wasting resources.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>