merge: refactor code that calls "git merge-STRATEGY"

merge: refactor code that calls "git merge-STRATEGY"

In the try_merge_strategy() function, when the strategy is "recursive"
or "subtree", the merge_recursive() function is called.
Otherwise we launch a "git merge-STRATEGY" process.

To make it possible to reuse code that launches a "git merge-STRATEGY"
process, this patch refactors this code into a new try_merge_command()
function.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>