merge-recursive.c: Add more generic merge_recursive_generic()

merge-recursive.c: Add more generic merge_recursive_generic()

merge_recursive_generic() takes, in comparison to to merge_recursive(),
no commit ("struct commit *") arguments but SHA ids ("unsigned char *"),
and no commit list of bases but an array of refs ("const char **").

This makes it more generic in the case that it can also take the SHA
of a tree to merge trees without commits, for the bases, the head
and the remote.

merge_recursive_generic() also handles locking and updating of the
index, which is a common use case of merge_recursive().

This patch also rewrites builtin-merge-recursive.c to make use of
merge_recursive_generic().  By doing this, I stumbled over the
limitation of 20 bases and I've added a warning if this limitation
is exceeded.

This patch qualifies make_virtual_commit() as static again because
this function is not needed anymore outside merge-recursive.c.

Signed-off-by: Stephan Beyer <s-beyer@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>