Build-in merge-recursive

Build-in merge-recursive

This makes write_tree_from_memory(), which writes the active cache as
a tree and returns the struct tree for it, available to other code. It
also makes available merge_trees(), which does the internal merge of
two trees with a known base, and merge_recursive(), which does the
recursive internal merge of two commits with a list of common
ancestors.

The first two of these will be used by checkout -m, and the third is
presumably useful in general, although the implementation of checkout
-m which entirely matches the behavior of the shell version does not
use it (since it ignores the difference of ancestry between the old
branch and the new branch).

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>