fast-import: grow tree storage more aggressively

fast-import: grow tree storage more aggressively

When building up a tree for a commit, fast-import
dynamically allocates memory for the tree entries. When more
space is needed, the allocated memory is increased by a
constant amount. For very large trees, this means
re-allocating and memcpy()ing the memory O(n) times.

To compound this problem, releasing the previous tree
resource does not free the memory; it is kept in a pool
for future trees. This means that each of the O(n)
allocations will consume increasing amounts of memory,
giving O(n^2) memory consumption.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>