Fix comment for GIT_FILEMODE_LINK

When we remove all entries in a tree, we should remove that tree from
its parent rather than include the empty tree.

When we want to remove the file, use the basename as the name of the
entry to remove, instead of the full one, which includes the directories
we've inserted into the stack.

0120000 is symbolic link, not commit

Instead of going through the usual steps of reading a tree recursively
into an index, modifying it and writing it back out as a tree, introduce
a function to perform simple updates more efficiently.

`git_tree_create_updated` avoids reading trees which are not modified
and supports upsert and delete operations. It is not as versatile as
modifying the index, but it makes some common operations much more
efficient.

Remove the now-unnecessary entries vector.  Add `git_array_search`
to binary search through an array to accomplish this.

Take advantage of the constant size of tree-owned arrays and store them
in an array instead of a pool. This still lets us free them all at once
but lets the system allocator do the work of fitting them in.

Instead of copying over the data into the individual entries, point to
the originals, which are already in a format we can use.